Changed my differential fluid last week. On the advise of Forum members, & Chevy parts adviser, I switched from GM Synthetic Axle Lube & 4oz. GM Additive, to Dexron LS, which already has the Additive mixed in.
Never had any problems....until today....after driving a bit & car was warm, every time I started in 1st gear from a red light, my tires would spin, & "Traction Control Activated" appeared on DIC! This never happened before I changed to Dexron LS & these were not aggressive starts.
Anyone else have this?

 

Strange issue... I changed mine to AMSOIL, and did figure 8's in the parking lot a few times to work the new fluid into the clutch packs in the rear end. Never had an issue here.

Procedure on my web site if you want to do it yourself. (www.toqueZ06.com)

I might suggest changing to a different fluid... maybe someone else knows whats going on though.

Toque

 

I've never used the Dexron LS but believe it has fairly high levels of friction modifier to prevent the diff chatter problems GM had when they came out the C6 diff. That fluid probably has too much additive for the C5 Z06 diff which does not need as much friction modifier as does the C6 diff. For historical perspective, when GM came out with the C6, they did not come out with a new diff fluid - until several years later, many TSBs on diff chatter - all from using the fluid and additive that had worked for years in the C5. Then they released the Dexron LS to fix the problem with the C6 diff chatter. Until your post, I've never really thought about the consequences of using this fluid in the C5.

What I suspect is happening is that due to the extra friction modifier, your diff clutch plates are slipping too much and your limited slip diff is not very limited slip. FYI, the spec for the C5 Z06 is for the diff to pass 75 ft-lbs of torque through it to the tire with grip. I check this often on my Z06 as I want max torque going to the tire with grip at the track. To check it, I lift one rear tire, with the gear in neutral and parking brake off (block front tire), I put a torque wrench on the axle nut of the wheel in the air and measure how much torque it takes to move the wheel. Should be over 75 ft-lb. I suspect you are way under that.

Before I spent money on an expensive diff rebuild, I'd try the simple thing first:
AMSOIL Severe Gear Extreme Pressure Synthetic 75w90 (Product Code SVGQT)
- 2 qts, no extra additive as this fluid has just enough to prevent diff chatter in the C5 Z06.

 

Common sense would dictate you are correct! Which brings up the question...why would certain Forum "experts"...& Dealer service adviser say to use the Dexron LS?
Now I have to drain & replace fluid again & use the proper GM Synthetic. And now I won't be sure how much additive will remain in the diff from the Dexron, since not all the fluid ever drains out.
Went thru this Crap with the new Dexron VI trans fluid....which at first GM said was back compatible with all Dexron III applications...until a year later when the addendum "except in manual transmissions" was released.

 

Sevice advisors, and parts guys at dealerships aren't that intelligent when it comes to stuff like this. I'm at a tech at a dealership and witness this everyday

 

Can't really answer your question about why others or techs would recommend the LS, but agree with both Keggs and your comments above about the Dexron VI vs. Dexron III (and its new part 88861800) which is an almost identical discussion to this one. Too many differement models, years, options for most techs to really understand the sublties involved so they just go with the corporate line - the fluids are backwards compatible. But in many cases they really aren't.

I personally wouldn't worry too much about how much additive is left when you drain the LS. Almost all the fluid does come out of the diff and whatever is left will have a very small impact.