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. Coach24

    Coach24 Rockstar 3 Years 5000 Posts

    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.
  2. pmf608

    pmf608 Epic Member 5+ Years 1000 Posts

    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.
  3. Coach24

    Coach24 Rockstar 3 Years 5000 Posts

    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
  4. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    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.
  5. Skippy

    Skippy Member 2 Years 100 Posts

    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.


  6. 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.
  7. the phantom

    the phantom Epic Member 5+ Years ROTM Winner 1000 Posts

    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!!!!
  8. ChevyBoy2009

    ChevyBoy2009 Rockstar 4 Years 500 Posts

    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 cant tell that i love Mobil 1 right?! :rofl:
  9. Skippy

    Skippy Member 2 Years 100 Posts

    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!

  10. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    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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