Supercharged111

So we've seen where thicker oil = bad for an engine not designed for it, and I'd be inclined to thinking the same goes for the rear, but the rear doesn't really require flow in the manner that the engine does. I'm wondering if I go with Amsoil 75W140, are there any downsides? I can't find the HTHS rating of it, which is the viscosity in cSt at 150*F, or around 300*F IIRC. I wanted to compare 75W90 to 75W140 at this temp, and compare that to the rating at 100*C, which I assume to be a standard operating temperature. I don't yet have trans and diff coolers, hence my curiosity for the heavier oil because I'd rather not grenade a diff in before I can get a cooler on it. We're talking about a much bigger jump in the big number than with engine oil, so I'm wondering if the viscosity at 150*C of the 140 is appreciably thicker than the 90. I'll call Amsoil tomorrow for the numbers, but I thought I'd prod for some personal experiences here as well.

froggy47

Redline 75w90 (includes the slip stuff). No issues 5 yrs, chg it every yr (probably overkill). I never heard of upping the diff oil wgt, mostly just guys switch to synthetic. Amsoil or RL.

If you are wtw or tt I would seek out those guys specifically.

moespeeds

Hey Frog are you running the shockproof with the red label or the MT with the yellow label?

redtopz

I used the 75/140 for several years with no problems. This year I switched to Royal Purple 75/90 and have no problems with it either. This is on 2 corvettes with and without diff coolers.

moespeeds

Just throwing this out there, I accidentally mixed Royal Purple 90 and 140 in a 50/50 ratio last season (don't ask!), and ran about 2,000 track miles on the car with no issues. I'm going to try the redline as Froggy recommends this time around.

travisnd

I run the Amsoil in the middle... Sever Gear 75W110.

froggy47

It has a yellow label includes the additive. I talked to RL and they said that for the stock diff that is best although I know some use the shock proof.

I could be wrong, but I think the shock proof has a formulation for built diffs with other than stock specs. But it works ok in stock diff. I would call RL they are good with recommendations for specific applications.

I have put over 35k miles on mine mixed use & it still grabs good on corner exit power down.

Supercharged111

Well I called Amsoil and they apparently don't rate the gear oil at 150*C like they do their motor oil. Guess I'll just go with the 75W90.

racebum

redline told me the same thing the 14.99 qt 75-90 with the additive. seems to work great. no chatter even on rainy streets and good traction out of corners

this is the stuff on the redline company website

http://www.redlineoil.com/product.aspx?pid=133&pcid=4

froggy47

larryfs

What temp does the warning light come on ? And is a cooler really necessary? I can't see the diff getting past 350-400 degrees, where good synthetic oil is still ok.

C66 Racing

Motor oil has a specification to meet at 150C, but to the best of my knowledge, gear lube does not which is why no one reports it.

My personal opinion is that a 110 or 140 is too high a viscosity and you'll lose net hp to the rear wheels. Been racing with AMSOIL Severe Gear Extreme Pressure Synthetic 75w90 (Product Code SVGQT) since 2004 and haven't lost a diff yet - that said, I do have my diff rebuilt periodically as the diff clutch plates wear out. Just had it rebuilt last weekend actually, first time in about six years.

While a good synthetic will not flash or choke at those temps, they will lose viscosity just like any other fluid. There is no temp below which is okay and above which is not okay. Risk increases as viscosity drops and fluid film strength drops. The speedometer is also driven off a plastic assembly on the diff and it will melt somewhere in that range. I have a tranny cooler on my car and I believe that helps keep some of the heat out of the diff as they are bolted together. On my C5 Z06 I have a tranny temp sensor and I believe the high temp warning comes in at ~274F. I don't get it with the cooler. To the best of my knowledge, there isn't one of the diff.

froggy47

Correct on the 274F I have had it come on. There is a way to hack the wiring & get it to display on the dic with a switch. I never bothered. Full synthetics in the tranny diff (and engine and p/s) seem to be enough on mine.

Supercharged111

My trans fluid had 5 events on it and it reeked, it was dark red. I did get the light once last year too, so I'm sure I was close to getting the light on a few occasions. The fact that ATF's ideal high temperature is what, 180*, means hitting 250+ cuts its life to nothing. I wonder about the Amsoil ATF, I imagine it resists breakdown better than the cheap stuff, but I think the cheap stuff would need to be dumped every 3 events so I'm wondering about the synthetic. I had a difficult time telling whether or not the gear oil was toast since it reeks to bad new anyway. I've heard if you have to pick 1, cool the diff. Makes me think that might keep the trans cooler too like what you alluded to with your trans cooling the diff C66, but I also think the gear oil takes the heat better than the ATF does, so you really would need both IMO.

C66 Racing

I'd say it depends.

I'm using AMSOIL Torque Drive ATF in the tranny and AMSOIL Severe Gear 75w90 in the diff - both have the same high quality synthetic basestock, so neither better or worse than the other in terms of "breakdown". I do think the diff fluid sees harder use, but it is fortified with extreme pressure additives (which is really what you are smelling with the diff fluid) because of this.

"Breakdown" is generally not well understood. Temperature in general doesn't "breakdown" a fluid, but it does lower is viscosity. The fluid does lose some to oxidation and at some point it will start to choke so there are temperature effects, but these generally occur over time or at pretty extreme temps. 250F isn't that extreme for most transmissions in demanding applications. The bigger impact is what is called viscosity shear down. As the fluid is put under pressure and shear forces, the molecules in low quality fluids physically break which lowers the viscosity. Though this is a diesel oil study it illustrates the point:
AMSOIL Nordic Waste Diesel Oil Field Study - Viscosity Shear Down (pdf)

I am thinking about a diff cooler. When I started racing in SCCA T1 (now T2) back in 03, I could only have a tranny cooler. With recent rule changes, both are now allowed.

Supercharged111

Yes, that shear down is what I always picture. I envision that the fluid has these "strands", and maybe that just applies to the innards of a viscous center differential as found on an AWD car, but over time those strands break down to shorter, less effective strands (correct me if I'm wrong here). I've seen where Amsoil advertises a substantially longer life from their fluid, so that's quite encouraging. That and you and others not blowing stuff up when serviced with Amsoil. I just serviced the diff and trans with Amsoil yesterday, went 75W90 on the diff. That gear oil poured damn near like motor oil, I really want some for the diffs on the tow pig now!

0Anthony @ LGMotorsports

Different oils, different viscosity.....all will effect temperatures and more importantly how the diff locks and unlocks.

With that being said...for most stock style Corvette diff's, 75w90 with one bottle of additive...

Want it to lock up sooner, don't put the additive in but the clutches will chatter.

C66 Racing

I do the same for both the tranny and diff in whatever I happen to have at the time as my tow vehicle.

Supercharged111

Yeah, I'm on the fence over the DexVI purge on my trans. I just did a modified transgo HD2 shift kit when I got converter shudder (and immediately parked the truck), seems to have fixed it. I then disconnected the return line from the cooler and put it in a bucket and proceeded to pump 4 gallons through my freshly serviced trans. Wow did I get a lot of **** out of that trans! Going to add a real filter and seriously considering repeating that purge with a higher quality fluid. The DexVI swap has hit or miss results when I search it out, even though GM says it supersedes DexIII. My trans (4L60E) is solid right now, so I'd be inclined to thinking it'd take good to the newer style fluid. On the other hand, it really doesn't get hot pulling the vette on my open deck trailer, so I do question the need for a higher quality fluid there. Once I hit the interstate it usually stays locked in 4th gear. When the weather gets cold, the truck downshifts a little more and I know the best place to start for that is the diff fluid.

CP Thunder

Both have always been allowed. Both of our cars came with them when built/purchased.