Joe C

update - I was just on BG's website. seems there are two 75W-140 gear oils, #752, and #753. the 752 does not have the limited slip additive whereas, the 753 does. so, we've got to ask, what gear oil did they use? I would tend to think that if they referenced any type of application data, they might have chosen the "additive" version. what I don't understand, is BG does offer product #751, which is a 75W-90 synthetic gear oil with additive. seems to me that would have been the more correct product to use.

https://www.bgprod.com/catalog/drive.../#bg-product-1

from what i'm reading, the 75W or 80W-90 is more for regular duty usage, whereas the 75W-140 is more for heavy duty applications. if the OP's fill does include the additive, i'm not sure i'd panic at this point.

Whaleman

UPDATE: The rear end started making noise within 20 miles. I did not take back to the shop. I don't trust them anymore. I am changing myself like I should have done. I have sucked out the old fluid. First question: I used a hand pump and got out about. 1 1/4 quart. I then taped a 3/8" clear hose to a shop vac and worked for a hour sucking. I got about 1/8 quart more. I can't seem to suck anymore. I am still 1/8 quart from what a full fill of 1 1/2 quart is. I guess this is the best you can do. The oil I pulled out really smelled bad. It seemed very thick compared to the Mobile 1. Stunk up the whole garage. Will fill with Mobile 1 75w-90 and one 4oz GM additive. Question 2: I see some recommend 2 bottles of additive. What is the correct answer. Thank You, Dan

Joe C

do it by the book and the book says 4 oz. mobile 1 75W-90 and 4 oz additive and you should be good to go.

Cjunkie

Mine took 4oz bottle of friction modifier and almost 2qt of gear lube.

Whaleman

What year (if it matters) Dan

Cjunkie

Probably not but mine is an 86.

Joe C

I don't think the C4 model year differs on the service requirements for the differential. this is my personal method for service --

using a (cheap, harbor freight) transfer pump, and slightly warm differential, remove as much fluid as possible. again, using the transfer pump, add approximately 1/2 quart of recommended fluid. add the contents of the GM additive (4 oz bottle) to the remaining 1/2 quart container, and transfer. add an amount from the second quart bottle to the maximum fill level. it should take roughly another 1/2 quart. approximate fill - 1.5 quarts (including the additive). install, secure, and torque the service plug. off the top of my head, i'm thinking 30 ft-lbs on the plug - double check and verify per your FSM -

Whaleman

Thanks, I was never sure if it was 1 1/2 qt PLUS 4 oz or 1 1/2 qt including the 4 oz. Dan

Joe C

"approximate" -- doesn't matter - just fill to the level of the oil fill hole. it might be a little messy, but not bad. there's no way getting around buying 2 quarts.

Cjunkie

This right here!

WVZR-1

20 miles? c'mon man

FOOLISH - unless maybe they reimbursed you their charges. Did they?

Did you have any conversation with them after you questioned their service here? If NOT I don't understand the "I don't trust them".

I don't believe I've ever seen the Mobil 1 "in the book" - the terminology changes year to year maybe but GL5 is the only spec I recall being consistent.

Joe C

the subject of that post was whether the OP should add one or two, 4 oz bottles to the differential. you need to re-read my response to the OP's post --

since the OP said he bought mobile 1, 75W-90, I indicated that, with the additive, and he's good to go. never implied mobile 1 was by the book.

Whaleman

Going to dealer today to get the additive. I will also take my fill plug and see if they have a parts finch. My plug had the red plastic warning sign but no washer of any sort. Need to find out if it had one. Dan

WVZR-1

there's "no washer' the tag should be floating on the plug and being a NTP when installed there's likely 2 - 3 threads exposed.

QCVette

Just another data point.

A couple of years ago I changed the fluid in my '96. I bought some synthetic (I think it was a Pennzoil product) that said for limited slip and did not need the additive. So I tried it that way. After a few miles of driving I found very bad chatter. The chatter was worst when very slow and turning hard (like in my driveway or out of a parking spot).

I sucked out some and added the GM friction modifier additive. It worked. Completely cured the chatter.

As a side note, I used a hand transfer pump and while lying there under the car, the tube came off the pump. I got gear oil all over. What a stinky mess. Hope yours goes smoother.

Good luck.

Joe C

"NPT" - , no washer - just a warning tag. BTW, how much torque on 2-3 threads exposed? . according to the 90 FSM, the drain/fill plug is 30 ft lbs torque, and the 96 FSM, is 9 ft lbs. that's quite a difference. off the top of my head, I don't know what size pipe fittings were used on the C4's. more research need to be done.

Whaleman

Well, I guess I got all the old out because it took the full 1 1/2 qt. Took for a short drive to do the figure 8's and all noise in the rear end is gone. Thanks everyone. Only funny thing was the first chevy dealer I went to (largest in area) did not carry the additive. Dan

Joe C

DGXR

For the record, you didn't have to pull out the 75w140 oil. It will work for your application, the initial weights (75w) are the same after all.

But the 140 is overkill -- you only *need* something that heavy if you are racing, or some other severe service.