I would REALLY like to try some synthetic gear lube in my '05 SE MT. Owner's manual says to use NISSAN (of course) 75W-85 GL-4 rated fluid. I have found some Castrol 75W-90 GL-5 fluid that my serv. mgr. feels will be fine.

I have used M1 in the crankcase for years and been very happy. Any thoughts from Bill or others? Thanks for the input!

If the service manual calls for GL4 DO NOT use GL5. I suggest you find another service manager.

Actually, my understanding (admittedly limited) is that GL-5 meets or exceeds GL-4 in all respects, such as API cert. SG, SH, SJ, SL, etc. Anybody have first hand knowledge of these spec's?

If it states GL-5 only, it has extreme pressure additives that will corrode the soft metals in the synchonizers. It's got to say GL4/5 service to meet both. Get Redline MT-90 for GL-4 syn geasr oil. Some people swear that mixing MT-90 and Redline MTL provides the best results.

Thanks for the info!

Try Royal Purple......I think you will like it better than Amsoil.....I did.
And yes, it as to be either GL4 or specifically state it meets both GL4 and GL5 specs.

As was noted before, find a new service mgr....