Don't panic. The GM Synchromesh gear lubricant (basically engine oil with an additive) is the factor lube and works just fine, 170K and still clicking off great shifts.
Good luck,
Mike
What GM sells now as "Synchromesh" gear lubricant, or whatever they call it, is not the "factory lube". The factory lube was a Havoline 5W30 engine oil which GM and ZF agree upon and that GM purchased from Texaco.
Problem is, Texaco stopped making that blend of engine oil several years ago. What "GM Customer Care and Aftersales" (used to be "GM Service Part Operations"--SPO--and before that "GM Parts Division") sells now is not the same. It is not known (but I think unlikely) that, almost a decade after the last ZF S6-40s were used in production, that GM went to the trouble and spent the money to have a lubricant supplier certify a new product, so GM probably picked a GL4 lube it was already selling and now markets that as the "factory" lubricant...and puts an obscene price on it.
That said, it is true that this "factory" lube meets minimum requirements (specifics unknown) for use in the S6-40. Some also like a Castrol 20W60 engine oil, which is, also, said to be approved by ZF for use in the S6-40, but others say it needs to be changed too often.
The best lubricants to use are either Red Line MTL (which is what the transmissions were raced on back in the late 80s and early-90s) or, even better, Red Line Light Shockproof.
I've used Light Shockproof for over a decade in two different ZFs with excellent results. Several years ago, I took one of them to Bill Boudreau (the "ZFDoc") for teardown and inspection. He was surprised, considering the miles on that trans and how it was used, that wear on some parts was no more than what you'd see just past break-in. He didn't like the pastel blue color, however, and calls it "Martian Blood".
