My brother looked at a 94 C4 today with the 6 speed transmission. The price is right and the owner just changed the fluids so he purchased the car for 5k.

When I was looking over the receipts he had with it, I noticed he has been using Mobil Synthetic 74w90 in the transmission. I thought these transmissions needed a lighter weight then this...is this something he needs to get on the ball and change?

Thanks

 

He should be fine with that lube. In general, it just needs a lube that's compatible with brass sychros (though I'd go for a synthetic -- as he did).

The OEM lube has been discontinued (from what I've read). The BMW lube is pretty heavy too. Plus, the measure for a gear lube isn't on the same scale as engine oil. IOW, a 75w-90 lube is closer to 30w engine oil than you think.

Any of the products (besides the BMW lube) will also be a 75w-90 anyway. Lots of people are running them.

 

Look here: http://www.zfdoc.com/

I'd get that Mobil synthetic out of there right away. Back when this tranny was new in
ZR-1s there were synchro failures because the Mobil synthetic at the time was too slippery.
Replace it with the BMW stuff according to zfdoc, or just get what replacement oil GM is selling today.

The original stuff you can't get anymore was a 5W-30 engine oil with friction modifiers for use in the ZF6, and it was made by Texaco.

 

That is true, but did they not use regular Mobil motor oil then, where the stuff today is 75v90 gear lube? (syn)

I have no idea myself, as I use amsoil, just throwing this out there.

 

Castrol is the only ZF-approved lube left. Keep in mind though, that ZF is NOT in the business of keeping this list up-to-date. Neither is GM for that matter. (In fact, I wouldn't pick the replacement GM lube. I suspect they're just pushing something across the counter at you -- because the "correct" stuff isn't available anymore!) What this means is there are acceptable fluids that haven't been tested for our application. And, they NEVER will be.

I don't remember exactly when the ZF rights were sold, but the info on this transmission is definitely into the try-it-and-see stage.

I've posted 2 or 3 extensive threads on the subject (after contacting ZF for guidance). After doing so -- and seeing what people are successfully running -- I came to the conclusion that modern-day synthetics should be fine. (Like AMSOIL, Royal Purple, Mobil shockproof, etc...)

Use the search function to get existing background on the subject.

 

Do not run the GM replacement oil. Take Zfdocs advise and run the BMW oil. I tried the GM stuff first and found lots bronze metallic particals in the oil. The BMW oil gave me a quieter and smother shifing transmission. Just my two cents.
93cruiser

 

I would say the gear oil is too thick for the ZF, I put Royal Purple in mine, and have had no shifting issues. But the only source for the fluid that originally came in the trans is BMW.

 

GREGGPENN is right with a 75w90 gear lube being close to a 30w motor oil, they are measured on a different scale. Also, the factory fill for the ZF6 was a 5w30 motor oil with some additives, not the BMW Castrol.

 

wow...seems to be no clear answer on this one...everyone has an opinion, if you use the search, it seems it makes no difference what you run, some people don't like it, even the BMW oil.....

 

The Royal Purple I used was their XPR line in 5w30. Their site lists their SAE 10w30 or XPR 5w30.

For what it's worth.

 

I do not agree with zfdoc's recommendation on the bmw castrol oil. I used this stuff and constantly found I was grinding gears. It was like the stuff was too slippery for the synchros. In some cases, I had to abort dragstrip runs because I awsnt able to physically move the lever into the next higher gear (it wouldnt go in). Also, when I drained the fluid, it was bronze with lots of sparkles in it.

Some friction needs to be present in there for the synchros to operate properly.

I got in touch with z_doc. He first told me that my master/slave cylinder were bad. I replaced them and bled them thoroughly, and the same issue existed. z_doc then told me I needed a rebuild (at a cost of thousands of dollars) or a new transmission. Well, when I put the gm factory fill 5w-30 (which is no longer available) back in there, all my problems disappeared.

I recently replaced the fluid with gm syncromesh and also had good results.

I do not agree with the suggestion on the bmw stuff.

***important note*** I am running the hurst short throw shifter - if that could make my situation a little different than the others.

 

duplicate post. please delete

 

The BMW oil made my car shift like crap. I put the GM oil back in and it shifts perfect. But I would be real curious too see how that matched against royal purple and some of the other aftermarket oils.

 

I did a search on the BMW oil....it seems quite a few people just don't like it...Think he may just leave the Mobil in the car

 

I've been using Amsoil since 2005 and haven't had any issues.

http://www.amsoil.com/storefront/mtf.aspx