differential fluid

[Christopherrr]

hey guy, I'm gonna yank out my passengerside axle shaft this weekend and since I gotta off the plate and drain the fluid I was just wondering what kind of diff fluid you guy recommend.

Thanks in advance....

[Joe 1320]

I like mobil 1 synthetic along with GM posi additive.

[Christopherrr]

I kinda figured that...Mobil...will it say Differential Fluid...or will be a thickness grade...I know where to get the GM posi additive...it's in a small bottle right...kinda redish in color.

[Mirroredshades]

85-90 weight.

Don't forget to loc-tite the locator pin.

It's easy to do if you want to save some money and do it yourself.

[Kevin - Blown 95 TA]

I like mobil 1 synthetic along with GM posi additive.

Me too! Although I use 2 bottles of the friction modifier.

[Aaron4TAMU]

I like mobil 1 synthetic along with GM posi additive.

When I got my gears installed, it says on the info sheet not to use synthetic. Should I really worry about this? I'm gonna be getting in there again soon to replace an axle shaft and crush sleeve, so I'll be refilling it.

[fastTa]

When I got my gears installed, it says on the info sheet not to use synthetic. Should I really worry about this? I'm gonna be getting in there again soon to replace an axle shaft and crush sleeve, so I'll be refilling it.

There has been alot of discussion as to whether or not it is good for the posi clutches to use synthetic gear lubricant. There are always going to be the people on other forums that are going to say "I used synthetic in my f-body rear for years with no problems at all."

But I can assure you that the posi clutches in the GM 10 bolt rear end were not designed to work optimally with synthetics.

I live in a fairly small town and I know several of the local GM dealership service techs and one service manager. They have all told me they cannot count how many times that customers have switched to a synthetic gear oil only to get clutch chatter and in some cases posi clutch failure. I've personally seen the results of the posi unit being treated to a dose of synthetic gear lube.

I also sought the advice of several f-body speed shops when we were putting the 4.10's in my 10 bolt rear in my 97 WS6. These shops were MTI, Combination Motorsports, and Thunder Racing, and they all said absolutely do not use synthetic gear lube in our rear ends. They also have replaced numerous posi units that have sustained pre-mature clutch failure due to the use of synthetic gear lube.

Just my $.02

[Aaron4TAMU]

Cool, thanks for the info FastTA.

[02Z28]

When I got my gears installed, it says on the info sheet not to use synthetic. Should I really worry about this? I'm gonna be getting in there again soon to replace an axle shaft and crush sleeve, so I'll be refilling it.

hmmm...you didn't go overboard again and bust a rim/axleshaft again, did you?...

[Aaron4TAMU]

hmmm...you didn't go overboard again and bust a rim/axleshaft again, did you?...

Nah, not this time. I think that a hard launch has my gears out of "ideal" alignment because the stock crush sleeve gave a tad. Anyone got any ideas on this? I know the ring/pinion/posi are good cause they are brand new and weren't making the noise at first. It's either that, or something in my tranny (dear god I hope not). As far as the axle shaft, it showed signs of wear whenever I got it (used). It should probably be fine for a while to come, but why not?

[Injuneer]

Interesting.... I've had my Strange 12-bolt filled two times over its 5 year life.

First time, the unit was brand new, and Strange had "forgot" to include the ABS gear that is machined into the "super-strength" posi unit, so they shipped a new carrier to a local shop that they had picked (with my approval) for a FULLY GUARANTEED replacement of the "wrong" carrier. The shop stocked ONLY Castrol synthetic gear lubes.

I've had the gears changed at Second Street Speed, and their standard replacement gear lube is Nelson Motosports Syngear II.

[Christopherrr]

wow...syn or no syn??? I guess I need to look into this more.

[Kevin - Blown 95 TA]

I wonder about it too. The Moser instructions said "We highly recommend conventional gear oil..."

Other things I know that support the use of synthetics & clutches...

Sportbikes almost all use synthetic in the gearbox - the clutches share the same oil. This was a controversy for a while, but the best oil for them available at the bike shops is synthetic.

Our Lincoln Mk Vlll has Mercon V (semisynthetic) in the auto tranny along with a friction additive.

Redline racing sells synthetic lube for the differentials with the additive already in it.

Maybe the trick with synthetics in clutched units is to also have a good dose of the friction additive, then it does OK ???

[Kevin - Blown 95 TA]

Nah, not this time. I think that a hard launch has my gears out of "ideal" alignment because the stock crush sleeve gave a tad. Anyone got any ideas on this? I know the ring/pinion/posi are good cause they are brand new and weren't making the noise at first. It's either that, or something in my tranny (dear god I hope not). As far as the axle shaft, it showed signs of wear whenever I got it (used). It should probably be fine for a while to come, but why not?

Ditch the crush sleeve for a Ratech spacer & shims, then it won't deflect into never-never land again. Of course, then something else might give...

[Joe 1320]

My posi has lasted 135K miles with synthetics. When the pinion seal went and the bearing shortly after, I upgraded the gears to 3.73s and still didn't need to touch the posi unit itself. It's had mucho quarter mile passes with sticky tires and no problems at all.