Hi all, I just changed my rear differential fluid yesterday. I've got a '94 with 104k miles. I bought it at 94k and have only now started the restoration.

So, diff fluid came out black and sludgy. I filled it with 4 oz of friction additive and a bottle and a half of SAE 80w 90 GL 5. As I drove it, the shifting seemed to hesitate even more; going halfway into gear and having to shift to another gear before entering first. It didn't want to let go of first gear either, getting stuck halfway and grinding. I was aware that the problems may worsen because the machine was relying on the friction of the thick oil to hold together.

The advice I got at the auto parts store was to redo the flush, because the new fluid may have cleaned out the varnish inside the diff, and there would still be some old fluid in there. They also recommended that instead of refilling with a bottle and a half of 80w 90, I should try adding 1 bottle of 80w 90 and 1/2 a bottle of synchro mesh MTF (penzoil) there's no grade labeled, but it says its GL 3 certified. They said it is a heavier weight and will provide some extra friction from clean oil, and that the weight mixing would be fine, and the fact that sychromesh is synthetic and the SAE is conventional would also be fine.

What would you do in my situation? It's an old, worn diff and was clearly neglected by previous owner. Tranny is probably in the same condition, and I would want a way to provide some extra friction so the loss is not too bad. (I can absolutely do a regular trans oil change and see what happens first).

I'm also getting mixed opinions on the tranny fluid for base model '94. General consensus is SAE 5w 30 engine oil is what to use, but some say MTF sychromesh 5w 30. More opinions on this would be great as well.

Thank you in advance, sorry for the long read!

 

It's not just any engine oil...which is why you're getting confused. It's absolutely 5W-30. It's absolutely not 'gear oil' like you'd normally expect for a manual transmission or differential. As WVZR-1 already pointed out to you in your other thread, Royal Purple lists the GM original part number as a cross-reference for their High Performance and Xtreme Performance 5W-30. From here: http://www.royalpurple.com/wp-conten..._Reference.pdf

But the two people who refurbish the transmission recommend any 5W-30 synchromesh gear fluid. Amsoil MTF fits the bill, as does Redline MTF.

Some info: https://www.corvetteforum.com/forums...f-6-speed.html
Some more info: http://www.zr1specialist.com/HAT%20Web/services/pm.htm
Even more info: http://www.zr1.net/forum/showpost.ph...26&postcount=8

 

So which would be best? Or does it just not matter?

Any opinion on the diff oil?

 

As WVZR-1 mentioned in the other thread, the one that gets the most "nods" (that is, approval) is Amsoil 5W-30 MTF. It's what I'm using as well. If it's good enough for Marc Haibeck and Bill Bordeau, it's good enough for me.

For the diff oil, I just use Mobil 1 LS 75W-90 because it's easy and inexpensive. Already has the friction modifiers and all that. I will point out that Marc Haibeck goes with Amsoil Severe Gear 75W-90 plus Amsoil Slip Lock limited slip additive.

 

awesome, thanks for explaining it twice, and slowly, to me. I'm learning a lot of this for the first time. Never realized oil was so complicated lol. Hats off to you, my patient friend.

 

Hey, no worries. A lot of stuff on the C4 is more complicated than it needs to be, because it used a lot of cutting-edge tech that...frankly never got used again. I mean, Chevy immediately dropped both the Dana 44 and the ZF6 after the C4 and went to the T56 and Getrag for the C5.

Wishing you the best of luck with recovering that car of yours! I know quite a few users 'round these parts have had similar labors of love. It'll be worth it!

 

Years ago I read that synthetic gear oil will eat through normal RTV sealant. As the rear case is sealed by something from GM. I don't know what sealant they used, so I instead just use mineral oil in the rear differential as I don't want to end up with a leak from say synthetic gear oil eating through the sealant that GM used.

I also remember at the time people saying some C4 Guru would say to put two bottles of that limited slip additive. I guess if your bottles already have the limited slip additive in it, then that would mean add one bottle of the GM additive in it.

 

Both of those are somewhat nonsense. Essentially half-truths.

On the RTV thing, it doesn't apply to us. Here's a post detailing where that comes from (a GM TSB): https://www.gmt400.com/threads/rear-...-5#post-355361

On the GM additive thing, the additive is a friction modifier. It decreases the friction of the clutches to make them quieter. This also impacts the performance of said clutches. So as a general rule for us, you only put in as much as is necessary to quiet the diff to your preference. Any more and you're unnecessarily sacrificing the performance of the differential.

 

What sealant did GM use on our rear differential cases? I know on mine there is a black substance that had squeezed out along the bottom. Is that RTV? I've read various things warning not to use synthetic gear oil in older vehicles as they say it can eat through the sealant.

 

No clue. But our vehicles aren't mentioned in the TSB, and as is stated, it's not the 'synthetic' nature of the gear oil, it's the particular additives in the one specific GM synthetic oil that were the issue. This then became the myth of "all synthetic gear oil is bad for RTV."

 

Ok, thanks. I'm going to be placing an order for some Amsoil ATF Signature Series transmission fluid. So I guess I'll also order a couple quarts of Amsoil Severe Duty 75W-90 Gear oil and a bottle of limited slip additive. It's really nice they offer their gear oil in a squeeze bag. Which with a clear plastic tube will make this a lot easier than last time. I'll cross my fingers that I don't get any leaks. As I had an older Camaro that kept leaking out the rear pinion seal no matter how many times I changed it.

 

A simple trick.
Take a clear hose and some compressed air.
Drill two holes in the bottle cap. One with a tight fit for the hose and a smaller for the blow gun.

GENTLY pressurize the bottle and the oil is forced up the hose.

 

I just wanted to cap off the thread here. I got the job done with little difficulty. I'll be doing it again after a short interval to help de gunk what was in there, and then add the fluid and friction compound recommended here. Torque specs for diff fill plug: 84-88=8-11 ft. Lbs, 89-95=30 ft. Lbs, 96=11 ft. Lbs. My thanks go out to you all!

 

What did you use to get the fill plug off? I seem to recall when I did it last time that there wasn't enough room to get a ratchet and Allen adapter to the plug as the floor storage buckets were in the way. And I used some vice grips to unscrew it. So I wasn't able to torque it down when I closed it up.

I think I may do like what you are doing is fill it, and then do it again shortly after to make sure you are removing as much crud floating around as you can. I've got a large pickle jar that has metal barbs on top, and I'd run a hose from it into the differential, and hook up a MightyVac that I rented up to the pickle jar and pull a vacuum into the pickle jar to suck the fluid out. I think this time though I'll rent a vacuum pump as I think I'd get a stronger better suction than the MightyVac hand pump.

 

I used a low profile 3/8 hex bit for a 3/8 ratchet. Stuck the ratched head on the bolt then fit the ratchet to it. I used a manual fluid transfer pump that screwed on to the bottle. Not too bad of a job, next time it wont take me all day!

 

Thanks. I'm going to see if I can find a low profile 3/8" hex bit.

 

I just had to get in there again today. Looks like I had to use a 1/4 ratchet with a low profile 3/8 hex head. Try applying some PB blaster 3x, then warm it up to full temp to expand the case. Then you can try wrenching it. You could also cut an Alan key down so you can fit a box wrench over it, and double wrench for more leverage. If all else fails, raise the ratchet handle with your floor jack.

 

I got a Plews 72-357 3/8" disc brake caliper hex socket. It's a lower profile than a regular hex socket. I did a test fit and I was able to get my 3/8" ratchet onto the differential fill/check plug with it.

 

I wonder if that 30 ft/lbs torque spec is a mistake? I know my FSM for my 89 calls for 30 ft/lbs. Though I find it weird how the spec is 11 ft/lbs for all C4s except it goes to 30 ft/lbs for 89-95, but then goes back to 11 ft/lbs for 96? My understanding is the carrier is essentially the same for all C4's? Did they change the plug for 89-95 which is why the torque spec jumps so high?

Just curious, where did you get those torque spec numbers from? I'm thinking on my 89 when I change the fluid in the rear that I'll just go with 11 ft/lbs. Anyone else know?

 

you are correct in your first paragraph. 11ft lbs for 84-88, 84ft lbs for 89-94, and 11ft lbs for 96, according to the Chilton manual. I would definately torque your 89 to 30ft lbs. But if you do go with 11 check it after each drive for awhile.

I confirmed the 30ft lbs in my 94 FSM.

94 FSM

Chilton 84-96