BierGut

Symptom/Issue:

'05 6 speed that had it's diff fluid changed one year ago to AMSOIL fully synthetic. Diff is not overfilled and does not leak. Drove the car for 6 or 7 months with ZERO odor. Following my first track event in the summer there developed a very strong diff oil odor that would drift into the car every 5 minutes or so on drive. Replaced the diff breather and it's improved the issue about 60-70 percent, but diff fluid smell is still there every few minutes. Inspection of the diff breather revealed the old one's rubber or plastic washer was essentially gone/melted, etc... So the decrease in smell makes some sense. But I still get an unpleasant odor and would like to alleviate it.

Questions:

Is this just as simple as the brand of diff fluid I used?

Is the smell an indication it's burnt and needs replacing?

Would Mobil 1 or GM's fluid not smell? (or smell as bad)

TIA for any thoughts or solutions if anyone has had same issues.

-- BierGut

American Heritage

We use both GM and Mobile 1 rear diff fluid in our shop. Never had any issues with smell.

needcheese

RPM transmissions recommend Valvoline synthetic. I personally stopped using amsoil motor oil. It just seem evaporate/disappear faster than mobil1. I never tried amsoil diff fluid. It might be doing the same thing.

C66 Racing

The smell is the extreme pressure additives and they are in all diff fluids. My wife HATES that smell.

There are several leak paths out of the diff. Don't know if they are as common on the C6 as the C5, and assuming your 05 doesn't have a diff cooler, the diff gets very hot at the track - to the point of melting portions of the diff in very long sessions at hot track days.

Left axle seal - common failure point on the C5 diff and won't get better with time.
Vent - It will release some fluid mist as the diff heats up. It can melt if the diff gets very hot.
Speedometer sensor - on the top passenger side of the diff is the sensor for the speedometer. When I lost my left axle seal in a race back in 04 and lost about half my diff fluid, my diff got so hot this sensor melted. Speedometer went away with about 5 laps left in the race. I finished the race, then rebuilt the diff. If your vent melted, I suspect you've partially melted this sensor as well and this is likely your vent path. It is all but impossible to see unless you drop the diff.

If you can smell diff fluid, you are losing it. At a minimum, I'd top it off prior to each track day.

After about 10 years of racing, last year I finally added a DRM diff cooler to my C5. I think the diff runs much hotter than I had formally thought.

Good luck finding and correcting your leakage source.