brassplyer

The subject pretty much says it. I'm contemplating draining/refilling the rear in my '80 since it's unknown how old the oil that's in it is.

Also, thoughts on rear-end additives - is the little bottle of additive mandatory? Most of the limited-slip gear oils already state they contain friction modifiers. On the additive as well, is there anything special about the Chevy product? Any you recommend over others?

Nowhere Man

GL5 80/90 gear oil. brand does not matter. woth 4 oz of posi additive

pws69

Mobil One Gear + 1 bottle of GM Posi Additive

derekderek

she prefers astroglide...

SunglassesGuy

this was discussed in detail over at another board I hang out on. https://www.thirdgen.org/forums/tran...-gear-oil.html

Nowhere Man

marketing and advertising is alive and well when they brain wash whats the best of anything. in the case of oil any oil they all have to pass SAE and API tests. so if they rate the same then one is no better then the other. anything else is marketing hype

Tampa Jerry

You could always go to your Chevy dealer. Jerry

HeadsU.P.

I agree w/ Nowhere man. Its funny sometimes when new products come along claiming to be superior or ending all previous issues from former products.
But then another decade goes by and the new kid on the block wasn't that great after all.

I will stick with the original because I never had any issues with good ole standbys. GM Rear Axle Lube has proven itself for how many decades now?

Two Lg bottles of lube & one Sm bottle of GM Posi modifier. A dealership would likely double the cost of that stuff, just because. So, you can go through Amazon or Muskegon Brake cheaper.

cv67

x2 your car doesnt need anything "special".

interpon

I used two 4 oz gm slip additive with whatever sale gear oil per the forum recommendation, then figure eights..no issues. Same situation didnt know what was in car and it was low..

Old Man in a C

I see what you did there.

jkippin

this wasn't quite what I thought it would be for a discussion. But I can tell you that anything with grit should be avoided...

DaGuvner

I sucked my old gear oil out with a vacuum pump rig I made, put a quart of mineral oil in and plugged gear case. Then, with the car on stands, started it, put in gear and let the the diff rotate at idle speed for several minutes to flush the old lube. Then vacuumed the mineral oil out ( which pulled more old lube out with it ). I then filled the diff to plug level with this Royal Purple product. https://www.oreillyauto.com/detail/b...ufacturer=true

It is a synthetic that includes the friction modifier. Not cheap but it will be in there for a long time.

brassplyer

Sounds interesting. The question that comes to mind is whether the coating of residual mineral oil over everything could possibly cause issues. Maybe first refill with a less-expensive oil, run for a short while and then suck that out and refill again with the oil you want to end up with?

brassplyer

"Then figure eights" - Can you clarify what this means?

brassplyer

Nowhere Man

Just want it means. Find a big parking lot and make a few figure eights

C3w4sp

Turning lock to lock causes the rear wheels to rotate at different rates. This causes the disks in the posi clutch pack to slip against each other. This works the lube with the friction modifier onto the faces of the disks in the clutch pack. This will allow the car to turn corners without chatter or clunking in the differential.

Figure eights are the best way to do this. It only takes a few.

Franks73

Hello. My 73 was off the road for many years. When I finally got her going, could hear a noise coming from the rear end. This very helpful forum advised me to remove the old fluid, add two 4 oz bottles of GM posi additive, and fill with GM rear differential oil. Driving the car in figure 8's, or tight circles in both directions exercised the posi clutches. What a relief that solved the problem!

GTR1999

The one critical point here is if you jack up the car and run it on stands you better have the 1/2 shafts parallel with the ground or you will bind the u-joints and break something. One guy I know locked up the diff and broke the posi case. Pretty expensive oil change.