[Hib Halverson]

I've done a lot of research over the years on the ZF S6-40.

First, be careful using GM's "synchromesh fluid". It was not intended for use in the ZF and it's viscosity may be too low for good durability.

The Castrol 10W60 is an acceptable choice but while Bill the ZF Doc likes it, that lube needs to be changed frequently.

The transmission lubricant used by GM for the portion of the ZF development which was done with transmissions in road racing cars was Red Line MTL.

I use Red Line Light Shockproof in my ZF and have been using it since '97. I change it every 36,000-40,000 miles. Periodic oil analysis shows excellent durability. Shift effort is reduced. It's calcium EP additive package has a positive effect on gear rattle, too.

For a number of years, I had a second ZF behind a 400 SBV8 in an old Chevelle which I also lubricated with Red Line Light Shockproof. Back about 1999, I took it over to Phoenix and had Bill take it apart and inspect the parts. They exhibited a very low level of wear, however, Bill has never been enamored with Light Shockproof because of its gel-based formula and it's color, a sort of "baby blue". Bill always calls it "Martian Blood".

Admittedly, Red Line suggests it be used in race only applications but for many years I've had excellent results with it in the S6-40 along with other manual transmissions such as the NV3500 five-speed and the Richmond six-speed.