it is a somewhat common practice to not use the modifier, as well as using fluid for an open diff (non LSD) to help clutches stay locked up, at the expense of needing to rebuild them more often.
Ok, so here is my REAL questions for you experts....will replacing diff fluid with a synthetic improve performance or do anything benefitial to you system versus just keeping the same fluid in there that was put when a gear swap was done years ago.?
Ok, so here is my REAL questions for you experts....will replacing diff fluid with a synthetic improve performance or do anything benefitial to you system versus just keeping the same fluid in there that was put when a gear swap was done years ago.?
IMO, After the initial period of 'breaking-in' the gears, I would have changed the fluid. All the same advantages to running synthetic oils in your engine or transmission apply here. You may not get in the car and go "Wow!" - but there is a difference.
i have read differing opinions on the syn vs conventional rear end fluid..... i think the concern with syn being that it is too slick. i would however, run a syn in the pumpkin. they are made to be run in there.
one big advantage: many synthetic 85-140's will flow into the bearings much sooner than a conventional 80-90. so one gets much better protection, as well as better flowing at cold temps. as i recall, it can take conventional 80-90 quite a bit of time to get into bearings.
i now switch all the cars over to syn's. it costs a little more money, sure. but one can pretty much fill it and not worry about needing to change the fluid (for service interval reasons). nice peace of mind.
good luck.