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C3 diff recommended fill? #5435092 05/21/20 12:56 PM
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cknight49090 Offline OP
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So i got the diff out it wasnt too bad but i dont want to do it again.Surprisingly the c clips where intact and it really didnt look that bad.I have 2 bottles of lucas (yeah i know) i think i bought it because it says no other additive needed if i believe them.I think it would be wise to go with synthetic so what is the best readily available choice?I have a tube of lubeguard and they say it will take the place of the GM additive?Since the axles or stub hubs wear in this application by metal to metal contact what if i put in a few ounces of redline break in additive for the added zinc and phosphorus? Thanks a head of time

5C813D1D-7159-492A-8904-B19DE302C5E6.jpegDA94BD14-76A4-4819-8D96-ABF08FF4368B.jpegB624AC60-F225-440D-8881-ABAB40DC0A5E.jpeg097FB02C-6033-4703-9FF7-153AAECECA2E.jpeg
Last edited by cknight49090; 05/21/20 01:00 PM.
Re: C3 diff recommended fill? [Re: cknight49090] #5435208 05/21/20 03:29 PM
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JeffKeryk Offline
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I use a good dino gear oil and the GM Whale Posi additive.


2018 Tesla Model 3, Medium Range Battery
2018 Lexus RX450h
2013 Lexus GS350 F Sport
2006 Acura TSX
2001 Tundra Access Cab, 1998 Accord LX, 1968 Corvette L36 Roadster, 1965 Olds 4-4-2
Re: C3 diff recommended fill? [Re: cknight49090] #5435365 05/21/20 07:31 PM
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cknight49090 Offline OP
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I think synthetic is the way to go especially with something you cant take the cover off like a normal differential,I only want to do this job once.

Re: C3 diff recommended fill? [Re: cknight49090] #5435373 05/21/20 07:40 PM
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joekingcorvette Offline
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Yes, synthetic for sure.


2005 Chevrolet Corvette
2014 Ford F-150 4x4 supercrew
2014 Jeep Grand Cherokee Limited (wifes)
2012 Honda Civic 4 door (my daily driver)
2007 Lund Tyee Grand Sport Mercury 115HP 9.8HP
Re: C3 diff recommended fill? [Re: cknight49090] #5435420 05/21/20 08:33 PM
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JeffKeryk Offline
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I would not use synthetic in any Corvette before the C4s.
Regular gear oil and 2 bottles of GM posi additive.
Put the additive in 1st.
Do a series of tight figure 8's in a parking lot afterwards.


2018 Tesla Model 3, Medium Range Battery
2018 Lexus RX450h
2013 Lexus GS350 F Sport
2006 Acura TSX
2001 Tundra Access Cab, 1998 Accord LX, 1968 Corvette L36 Roadster, 1965 Olds 4-4-2
Re: C3 diff recommended fill? [Re: cknight49090] #5435465 05/21/20 09:23 PM
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cknight49090 Offline OP
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Listen I respect your opinion but synthetic has to be better its evolution what your saying just doesn't make sense.The lubes they had when these cars where made is why the stub hubs wear and that's what I am replacing and don't want to do it again,Even in a C1 or C2 even more so I would think a synthetic would be better in every regard I don't claim to know everything could you shed some light on this?Its like telling me to drain the DexVI out of my trans and put back In DexIII.That is why I am asking I want to use the correct lubricant.

Re: C3 diff recommended fill? [Re: cknight49090] #5435770 05/22/20 10:43 AM
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JeffKeryk Offline
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In the Corvette world, there are a few true experts. One of which is Gary Ramadei. He rebuilds rear ends and offers his expertise to all.
While synthetic oils are better than dino in almost all respects, this may be an exception.
Synthetic gear oil will probably work fine in your C3. Most of these cars are hardly driven and not really beat on.
I am running the GM additive and (probably) Valvoline gear oil in my rebuilt Corvette rear.

Here is an article by Gary for your reference:
Diff Rebuild

Near the bottom is his fluid recommendation.

Good luck!

68 side.jpg
Last edited by JeffKeryk; 05/22/20 10:44 AM.

2018 Tesla Model 3, Medium Range Battery
2018 Lexus RX450h
2013 Lexus GS350 F Sport
2006 Acura TSX
2001 Tundra Access Cab, 1998 Accord LX, 1968 Corvette L36 Roadster, 1965 Olds 4-4-2
Re: C3 diff recommended fill? [Re: JeffKeryk] #5435776 05/22/20 10:54 AM
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cknight49090 Offline OP
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Thanks I will check it out..

Re: C3 diff recommended fill? [Re: cknight49090] #5436025 05/22/20 04:27 PM
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BlakeB Offline
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I also vote for Dino 80w90 or 85w140 if you are going to be beating on the car. In my experience, mechanical LSDs do not play well with synthetic gear oils. Almost all of the professional shops that I know of that do rebuilds recommend 85w140 in warm climates or 80w90 in cold climates. If it were mine, I’d drive it about 500 miles and drain the break in oil, followed by another drain and fill in another 5,000 or so miles.

I’ve read that while oil temperatures in an axle filled with synthetic tend to be cooler, the hot spots on the gears themselves tend to run higher with synthetic gear oil. Supposedly it has something to do with how the more viscous Dino lube clings to the gears and thus is more effective at removing heat.

I think you’d be hard pressed to kill the gear set with either oil and it is more about which type of oil (conventional or synthetic) is going to behave the best in regards to the posi unit.

I had a mechanical limited slip in my 07 Tacoma that would not play nice with synthetic that was rated for limited slip differentials. I put in some Lucas 75w90 that was supposed to be good for LSD and it chattered terribly, another bottle of limited slip additive did not help so I drained it and filled with Dino 80w90 and a bottle of limited slip additive and had no more issues. Of course your diff may be totally different and do just fine with synthetic.

Re: C3 diff recommended fill? [Re: cknight49090] #5436141 05/22/20 07:07 PM
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cknight49090 Offline OP
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Thanks the diff has 99,000 on it is very tight no backlash at all i think i am just going to replace the stub hubs which are used oem with no wear and call it good.Draining and refilling isnt an easy task nor horrible either i would have to suck out and then replace to there will be some left.With the parts all being broke in i think i will be good and maybe use half 80w90 and half 85w140 or a full bottle of the later and the other 1/3 with the 80w90.I was thinking about adding a magnet to the housing or somewhere probably should expoxy it in.

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