Who makes the best gearbox oil?

[Mark300zx]

Would like to put in fully synthetic and would like some opinions on what is the best out there?

[stevieturbo]

There is no best.

Each and every application is different, and may require a different oil.

But Millers make good stuff

[350Matt]

Castrol SMX-S

Excellent stuff

[Busa_Rush]

The correct grade is probably more important. After that Castrol or Silkolene perhaps ?

[Chris.B]

Depends upon your application.
Note though - some boxes specify GL4 not GL5, as GL5 is slightly too acidic for the bearings (eg BGH recommend GL4 synthetics).

[V10Mike]

The choice between GL4 and GL5 also depends on the synchromesh -for many older boxes (eg Muncie, T10) GL5 is too "slippery" for the synchro to work correctly, giving baulky shifts.

[Mark300zx]

Thanks for all the replies, it is for a gearbox that requires GL-4 and an oil weight 75-90.

[untakenname]

Redline or Royal Purple if you want a proper synthetic oil for high powered applications, made such a difference when I changed from mobil 1.

[MGJohn]

untakenname said:

Redline or Royal Purple if you want a proper synthetic oil for high powered applications, made such a difference when I changed from mobil 1.

Hmmm .... interesting, can you please define how "made such a difference" ... that way we all can learn.
.