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
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.