the vehicle should tow 6000lbs. with normal sluggish tow effort. now I would look to install a transmission cooler. also I would only tow in 3rd gear. you have to use 89-93 octane when towing. replace fuel filter/check fuel pressures , new air cleaner . use synthetic oil.
check for a restricted exhaust. could be clogged cat converter
I used to tow with a 1983 307cu car a 4200lbs boat . towed real good , but I had a huge tranny cooler and with a carb a ram air setup . also a 3 core radiator. never had a problem 380,ooo miles sold vehicle. 17yrs of use.
I don't see how changing to synthetic oil is going to increase his performance. The only thing that will do is increase oil life