While I have the 82 up on the lift and the exhaust off I want to change the differential oil.
How much oil and how much limited slip additive do you need?
I find the limited slip additive Genuine GM Fluid 88900330 Limited Slip Axle Lubricant Additive 10-4003 - 4 oz. on .

I find several gear oils so there is some confusion there.
AC Delco 92184900 GM Original Equipment 10-4032 75W-85 Axle Gear Oil - 1 L on .
AC Delco 19352759 GM Original Equipment 10-4108 75W-90 Dexron Gear Oil - 32 oz on .
AC Delco 88861800 GM Original Equipment 10-4033 75W-90 Manual Transmission and Transfer Case Fluid, 32 Ounce, on .

Thanks
JT

 

2 large bottles of GM axle fluid.
2 small bottles of GM Posi modifier.

You can do all your batch mixing on your work bench.
Pour mixture into a small squeeze bottle (condiment) that will easily fit up in the diff fill port area.

 

Which axle fluid? I found 3 different ones...

I have one of those suction things with a hose so getting the fluid out and back in "should" be easy.

JT

 

This is what the resident differential expert (@gtr1999) recommends.

 

While today I would use Lucas that GTR1999 has recommended, I used GM rear axle (AC Delco 92184900 GM Original Equipment 10-4032 75W-85 Axle Gear Oil) in my '72 a couple years ago with a couple bottles of the Posi modifier.

Be careful with any hose you use to push fluid into the differential -- there have been stories here about the hose coming off the container and getting "lost" inside the case. There's no cure for this other than disassembly; make sure the hose is ~10" long.

 

When I regeared my diff one bottle of additive and the rest was Valvoline 80W-90. Ditto for my Brandy new Strange engineering Dana S60 that when into my Cuda. Any name brand 90 weight is good in my book. And I wouldn't buy oil from Amazon. I don't trust anything they sell to be The real McCoy.

 

So I'm assuming it's not critical what brand or weight you use so long as you use a decent brand?

JT

 

The weight is important, the brand not, and it has to say for limited slip differentials.

 

if you live and drive in very cold climates, you can use an 80W-90 weight but if you are like most and drive only in warm weather, then the 85W-140 is a better choice.

There are thousands of posts on the topic of oils (mostly motor oil), many with anecdotal support, many with emotional support, and a few with solid science-based support. As the saying goes, opinions are like a##-holes - everyone has one. Do your homework if being able to defend your choice is important to you.

For me, if Gary Ramadei (@gtr1999) recommends the Lucas 85W-140, that’s the only support I need. It is a mineral-based (non-synthetic) lubricant.

 

Look at it this way:
If the std GM lube got you 50 yrs out of it, why wouldn't the same product go another 50?

I am a firm believer that if a reservoir (oil, trans, diff, PS) came from the factory with non-synthetic,
then by all means stick with non-synthetics.

The exceptions would be complete rebuilds, new parts of course.

For what it's worth, Amazon is selling legit GM products. They buy in huge-huge quantities to sell at a discount price and still make a profit.
If you don't believe its legit, feel free to go buy it at the dealership for double or triple the costs, exact same product.

Which part number on Amazon? I don't remember. But I use it all the time w/o issues.

 

Just for fun I thought I'd look it up in my gen. GM service manual.

 

Interesting, my 82 FSM doesn't mention anything about limited slip gear oil but does mention the additive.

 

80/90wt gear oil will work. Posi or Limited slip is best used by the small 4 oz bottles. I use GM additive.

I have been using Lucas 85-140 for decades. Goes back to a long conversation I had with Tom Watt many years ago. For those who have spoken with Tom, the bottom line is "Dude, the 85-140 Lucas works". It is not called out in any manual, literally 1000's of diff's I have built use it. Many here use it, I have never had any issues with it.

Some will use oil with the additive in it, use expensive brand names. Some will work, others may not. I used a Valvoline 90 wt ger oil with the additive in it I bought from NAPA 40 years ago. It caused posi chatter, the diff at the time was a stock 69 Eaton that I did not rebuild yet. I sucked as much of it out as I could-I didn't tap the housing yet either, installed the GM additive sold at the time, and 90 wt gear oil, no more issues until I did rebuild it.

Now, an 80-82 Dana doesn't use spring packs like a stock posi. They use Belleville type
washers to preload the clutches. The Dana parts are smaller in size than the iron diff's.

The Eaton posi's used several revisions of clutches over the run and into the aftermarket. Today, they come with Fiber coated clutches, which I don't typically use. They will not chatter as the 2 types of steel clutches may, instead they smudge away at every turn. They will work but it is not recommended to use synthetic oil with them, as they may slip too much. Again, the 80-82 is different.

 

Thanks for the information Gary. While it was fun researching the original oil using something that works is the key.

JT

 

I have a '78 very high mileage (over 200K). Is the Lucas 85-140 the best option for me and is it used alone, no additive?

 

You need to add a bottle of additive and Lucas is the correct choice.
Have you checked the differential side yokes for in and out play?

 

200k on an original 78 diff or one that has been previously rebuilt?
If original, you got your monies worth, since those had case hardened axles as all C3 axles had. Those lasted about 40-50k miles before the hardening was gone, leaving a soft core that wears down. As Mel suggested, check the axle endplay, anything over 040" you will need more than an oil change.
I like the Lucas oil and it has worked fine for me and the 1000's of others who have used it. You can use 90wt as well. You need to use the additive with a posi diff, which yours should be. I say that since all vette from 1970 were posi but I have had in rebuilt diffs where an open case was used. Not many but I have seen it.

 

Thanks for the advice. Yes it is original and no, end play has not been checked so it is quite likely well worn. Eventually I will end up spending the money on rebuilding but for now I'm hoping to just quiet the chatter and binding I feel while slow speed turning.

 

If the axles are worn down to the point they hit the differential seal lip, you may end up wrecking the differential housing.