ZF Transmission Recommended

The 6 speed manual
transmission in 1989-1996 Corvettes was produced by German automotive
components company ZF Industries http://www.zf-group.de/. The Corvette
Owners Manual clearly states that this transmission does not need service,
but the transmission fluid level should only be checked and kept full. A
bit of research confirmed that ZF does not share this recommendation with
GM. Instead, ZF recommends that the transmission oil be changed every
30,000 miles.

The transmission requires GL-3 rated gear oil.
Gear oil is categorized from GL-1 to GL-6, and the ratings are not upward
or downward compatible (i.e., it is not okay to use GL-2 or GL-4 in a
transmission that requires GL-3). ZF is recommending to use either the GM
factory fill (GM Part # 1052931) or Castrol Formula RS Synthetic. The
Castrol oil is not available is stores. You have to contact a Castrol
dealer to get information on where you can buy it. It is important to note
that no regular motor oil meets GL-3 specs. Also, none of the gear oils
from Redline are suitable, because Redline only offers GL-4 and GL-5 gear
oil.