I am going to be changing my rear diff fluid, and I purchased 2 quarts of Mobil 1 synthetic fluid. The rear is 3.42 posi. Should I also add the GM posi additive? The additive says it should only be used w/ convential oils, and Ive read that it is not needed with synthetics. Any help would be great!
Thanks...

 

It's definitely needed with synthetics. Put it in first, then fill it up with gear lube.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Your brush is too broad. It is not needed with AMSOIL Series 2000 75W90.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4. 2.93 limited slip. Cat-back from '91 GTA, ZZ3 intake, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam, ported World 305 heads, Hooker headers & y-pipe, 3" Catco cat & 3" cat-back).
57 Bel Air, my 1st car. '66 396, 9.7 CR forged TRWs, Weiand Action+, Edelbrock 1901 Q-Jet, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 600 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).

 

Ok, thanks.. I have the Mobil 1 already, so I'll put it in.

 

I used mobil one sythentic gear oil in the rear of my z28 and noticed it would make weird clunking noises if i turned sharp at a low speed. This was due to not having any additive in it. i ended up changing the oil again and using the GM stuff and it worked perfect

 

i have a limited slip, i added mobil 1 synthetic, with NO additive, no noise or problem at all. but thats just my situation.

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88' GTA 350 MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass

 

Always add the additive!! It will not do any harm!

 

ive been running for 30k with mobil 1 with no additive and have had no problems what so ever