Gear Oil Question

Sanco213 · May 5, 2009

i am building the rear in my 2003 gt. i purchased an eaton posi unit and moser 31 spline axles. the paperwork that came with the locker says to use 80w-90 mineral based gear oil (and 8 oz of limited slip additive). i am not sure exactly what that is. is it everything that is not synthetic gear oil? why souldnt i use synthetic? isnt it better? any input would be greatly appreciated...

squeak93 · May 5, 2009

It doesn't matter synthetic or not as long as you use the same weight..... Go to walmart or any parts store and just look for that weight in rear oil. Go to FORD for their friction modifier. Oh, and prepare yourself as it stinks to high heaven....

Stan Weiss · May 5, 2009

Why would you not contact Eaton and find out what the unit was manufactured / engineered to work with?

Sanco213 · May 5, 2009

squeak93 said:
It doesn't matter synthetic or not as long as you use the same weight..... Go to walmart or any parts store and just look for that weight in rear oil. Go to FORD for their friction modifier. Oh, and prepare yourself as it stinks to high heaven....

well... i know that there are a few different types of oils.. there are petroleum based.. synthetic.. and mineral based... and it also says to use a GM limited slip additive... so what are some mineral based gear oils??.... im still confused about it..lol...

Night Shifter · May 5, 2009

either go with motorcraft or mobile 1 synthetic.. and be sure to go to ford and buy the friction modifyer...you'll thank us later

on a side note, before 2001 Ford spec'd 80W-90 gear oil, mid 2001 they started specifying 75W-140 synth. But regardless the year, i'd go with what the paperwork suggests

Cobra Jay · May 7, 2009

Run Royal Purple Max Gear 75W90. Already has additives in it so no need to buy friction modifier.

Max-Gear is an ultra-tough automotive gear oil. It’s designed to maximize power and provide unsurpassed protection to heavily loaded gears. It makes gears run smoother, quieter, cooler and longer without overhauls.

Max-Gear outperforms other gear oils because it combines the highest quality synthetic oils with Royal Purple’s proprietary Synslide additive technology.

Max-Gear is recommended for use in truck, motor home / RV, and automotive front or rear differentials, manual transmissions, and lower gear units of marine engines that specify use of an API GL-5 or GL-4 fluid. All viscosities of Max-Gear are formulated with hypoid friction modifiers necessary for use in clutch or cone type differentials. No additional additives are necessary.

Royal Purple Max-Gear

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