Gear head said not to use synthetic in diffs and this is why!!

Discussion in 'Maintenance & Upkeep' started by the phantom, Aug 7, 2012.

  1. the phantom

    the phantom New Member

    I was talking to a guy that rebuilds front and rear differentials for the past 30 years and he told me that synthetic oils are great. But not for the differentials. He said the reason why you shouldnt use it is because conventional gear oils push the contaminates to the side walls of the diff cases and it sticks there not allowing it to be continually ran through the gears and bearings. He said that he has seen bearings shot after about 50-60k miles because of this. He said that synthetic oils are the best for any system that is filtered but diffs are not. And the properties of the synthetic blends do not allow any wear contaminates to stick to the sides like the conventional oils. I guess if you continually change the oils maybe this statement may not apply. But then at what interval would be acceptable to not allow any of the contaminates to wear on the bearings ect. Just thought I would share an opinion.
  2. RayVoy

    RayVoy Active Member

  3. the phantom

    the phantom New Member

    That is a lot of testing that is for sure. I guess my only thought on the tests are that it really doesnt put any of the oils through its real life scenario such as the time factor involved. If these tests could be conducted over a period of 4 and 5 years including the temperature changes in that amount of time I may lean more to agreeing that the synthetics do what they say. I think I would lean more to using synthetics in my diffs when I change my gears soon and just change the oil out often. Afterall I did buy a rear diff cover that has a drain plug in it.:great:
  4. RayVoy

    RayVoy Active Member

    GM diffs (especially the front one), have a habit of picking up water, changing the fluid more often than the recommended interval is not a bad idea (unless you get it done at a GM shop).
  5. dpeter

    dpeter New Member

    They push contaminates to the side walls of the diff case!!?? WTF and OMG. You know my dad had a car that only took a key to run. I know this for a FACT because I never saw him change oil or put gas in it so it had to be the key. I wonder what ever happened to that car. I think I was about four or five when it went away and then he got this truck that he was always putting gas in. How is it that old saying goes--- Believe half of what you see and none of what you hear.
  6. Pikey

    Pikey Moderator Staff Member

    My gear guy said the exact same thing when I had the front and rear diffs redone on my 03 yukon xl. He runs conventional oil with a friction modifier. He refuses to put synthetic in a diff. I said "I will buy the synthetic oil to put in the diffs when you are done" He responded, "You can buy it, but there is no way in hell I will put it in, you are welcome to drain my oil out and put it in yourself, but I will not honor my warranty".
    Last edited: Nov 30, 2012
  7. RayVoy

    RayVoy Active Member

    Well, there are still guys who insist on changing engine oil every 3,000 miles..........................



    .........hmmm, what did I just start:rofl:
  8. the phantom

    the phantom New Member

    So if you drain the oil out, then pull the diff cover, obviously all of the contaminates will not be suspended in air. So how do you find out how true this is. Remove the entire rearend out of the vehicle, put it on a bench and pull the cover off facing up and look inside with the oil still in it??? suck the oil out and see if the contaminates are against the walls of the housing? what prevents the contaminates from sticking to any of the other parts inside that dont move. I dont know. It sounds like a possibility that it may be true. But we could also argue that synthetics are so good that there are no wear contaminates to worry about.
  9. dpeter

    dpeter New Member

    "But we could also argue that synthetics are so good that there are no wear contaminates to worry about."
    I'm going with this one:great:
  10. tbplus10

    tbplus10 Super Moderator Staff Member

    When you drain the oil out if you strain it you'll find your contaminants, right in the oil, just like conventional oil.
    If this mechanics theory were true you wouldnt need a filter on your engine, all the contaminants would be stuck to the sides of the oil galleries, I've broken open quite a few engines, gear box's, and axles running Synthetics and none of them had anymore contaminants sticking to the sides than there was on parts using conventional dino oil.
    When I used to work on Jets we had huge oil pots for the main induction gearbox's that unbolted from the bottom of the gearbox that you dropped straight down then tilted to drain the oil into a bowser, we used synthetic oil for all the engine lube and after dropping over at least a 100 oil pots in a 20 year career I've never seen what that mechanic describes happen, the contaminants were always mixed in the lube.
  11. Coach24

    Coach24 New Member

    Hmmm. Time to search for more detailed and long term tests to compare , me thinks. good arguments on both sides just not sure which I will follow at this time.
  12. pmf608

    pmf608 New Member

    I'm a bit skeptical, so I'll stay neutral on the truth of this claim, but I'll personally stick with GMs spec of using synthetic oil in my rear axle. As far as the front diff goes on my 03 Avalanche, the original GM spec was for non-synthetic gear oil, but I'll use synthetic on my next change because GM put out a bulletin saying that using synthetic would help reduce the damage it will do to itself, especially in the cold winter climate we have here.

    I'm really just a fan of synthetic oil overall though.
  13. Coach24

    Coach24 New Member

    I see both sides of this issue and wonder am I on the right side, the fence or the left side. Currently on the fence but PM is coming due so I need to make a decision
  14. tbplus10

    tbplus10 Super Moderator Staff Member

    With the cold temps you regularly see in your part of the country Synthetic lubes are probably the best, oil sludges and will even freeze at a certain temp (depends on the grade your using) the viscocity changes drastically when you get near 0deg, you wont have these issues with synthetic as its formulated to with stand colder temps.
  15. Skippy

    Skippy New Member

    The logic here from your mechanic is slightly flawed. Given that the gear oil suspends particles, and particles will cling to any material surface they come in contact with, if you follow your mechanics logic that the contaminants are "pushed" out of suspension, they'll also be pushed onto the gear surfaces, which would be worse than being simply suspended.

    I can completely see a logic trail that would lead a mechanic to conclude if crap is stuck to the sides of a gearbox, it must be better than having it float in the oil... That would only hold true if the crap wasn't actually the oil. Conventional oils breakdown far faster under sheer pressure than synthetics. When I've cleaned differentials, I've noted the same crap on the sidewalls of the insides that your mechanic notes. I've also seen synthetic oils not produce nearly as much of that junk.

    It's important to not that this is a sealed system with NO blow-by (such as in an engine), the only contaminants in gear-box oil are produced by the gearing, the oil, and any water that finds its way in (usually from a submerged axle, requiring a swap of fluids anyway).

    Because conventional oils breakdown faster than synthetics under massive sheer pressures, when your oil breaks down during the gear contact, the by-product is that your gearbox will wear faster. This means you have contamination from broken down oil AND particles from the metal wear. It may not even be visible to the eye 40K miles, but minute particles worn from the metals will be higher in convention over synthetic.

    I'm with dpeter on this one. Quality synthetics will result in less wear over time, and do a better job protecting your diff from both sheer and heat damage. Increased friction increases the heat output... conventional oils will simply NOT be as cool as synthetics over the whole life of the oil.

    I find your mechanics comment that he has seen bearings fail "Because of this" an interesting postulation. Concluding causation in an environment with a myriad of parameters is difficult at best. For example, how does your mechanic KNOW it was the synthetics that caused it? Could it have been because the synthetics replaced the crappy conventional oils and resulted in so much crap being dislodged from the sidewalls that the oil couldn't do it's job (meaning if conventions had never been used it'd be better?) Or what if there were minute fluctuations in the bearing fabrication process that were simply exposed during normal wear and tear. Was it simply a defect part?

    Basically, having a few events (over 30 years!) where synthetics were used and bearings failed does NOT make a conclusion. In fact, he may simply be LOOKING for synthetics during early part failure that is actually a result of a failed PART, not fluid!

    When folks are looking for evidence to support their already forgone conclusions, it's been my experience they'll usually find it. :)

    That being said, I believe the minimum expected serviceable life of your differential and gear box will be just fine using either quality conventional or quality synthetics. I believe high quality synthetics may make it last longer, run cooler, and have less friction total resulting ever-so-slightly better MPG, but the minimum lifespan will be met either way! :)

    If your mechanic won't warranty his work because he believes synthetics are poor substitutes, I personally would stick conventional fluid in there. You'll save a few bucks at the changes, and it becomes his problem to deal with if there is premature failure.

    Cheers,

    -Skippy
  16. steved

    steved Former Member


    Wow, based on this logic I wonder how my last truck (450HP+, 800TQ+, manual trans, towed regularly) made it to 270k with synthetic fluids in the differentials. Does this guy realize that almost all new cars come with some grade of synthetic in the differentials from the factory? How many of them ever actually get changed and they live a lot longer life than 50k?

    It is more likely his experience comes from a failing differential that someone put synthetic in as a band aid, and because it failed with the synthetic fill, it must have been the result of using the synthetic fluid...not that the differential was on its last leg to begin with.

    On edit: another thing to keep in mind is that not all synthetics are equal...there are good ones, there are bad ones. But no different than the conventional oil choices.
  17. the phantom

    the phantom New Member

    Im leaning towards putting synthetics in my diffs when I have my mechanic put my new 4.56 gears that I should be receiving monday in my truck. This guy I was talking to claiming this was NOT my mechanic and had a booth setup for selling gears and was just giving me his recommendation. Needless to say he did not impress me enough to buy the gears from him and I ended buying the same ones he was selling from a different vendor. Although with the test info that RayVoy had linked and some of the other opinions here I feel I learned from this thread. THANKS GUYS AND GMTC!!!!
  18. ChevyBoy2009

    ChevyBoy2009 New Member

    I will always run synthetic oil in my diffs...i use Mobil 1 Fully Sythetic 75w-90 in my rear end and cant remember what i put in the front of the last truck...when my new truck comes due for rear diff and front diff change,TRUST ME i will us sythetic oil...and i run Mobil 1 FS 5W-30 in the motor...you cant tell that i love Mobil 1 right?! :rofl:
  19. Skippy

    Skippy New Member

    I'm a fan, too. Though, I get a kick out of the various arguments about which high end products are that Nth degree better. Seriously... If you use synthetics and follow the standard change schedule, you'll never come close to getting to the point where it'll make a difference. I love Mobil1, because it's a GREAT product without the insane prices of the other guys... Though.. >i do run Royal Purple in my rear diff... only because Mobil1 never seems to be in stock when I do my rear diffs!

    Skippy.
  20. RayVoy

    RayVoy Active Member

    I've got RP in both of my diffs as well, but after reading the comparison testing, in that article I found for post #2, I might have 2nd thoughts about continuing to use it. The article did say all products tested met the industry requirements, so it is probably a good product to use, just that some of the others tested so much higher.

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