Manual transmission oil

[glennm01]

The shop guide recommends the use of SAE 30 or 40 API CC engine oil in the manual transmission for "vehicles operated continuously at high speeds." I picked up a gallon of Shell Rotella SAE 30 today thinking that would do the trick, but after reading up some more on the whole API thing, my head hurts, and I'm doubting that this is the right stuff:



Anyone else out there running engine oil in their manual gearbox, and if so, what do you use? This is for an '04 FG649, btw.

 

[Howard70]

Hello Glen:

Our FG is newer (2017) and has the Duonic transmission rather than a manual. However, the gearbox portion of the transmission and the transfer case lubricant recommendations from Fuso have evolved from 80 - 90 weight gear oil to motor oil as you found and finally to Mobil Fluid 424 (some confusion exists as to whether or not the MF 424 is recommended for the transfer case). We originally ran the gear oil as that is what the documentation originally provided by Fuso with our truck specified. However after conversations with some Fuso dealers who reported on later service bulletins from Fuso we debated between the motor oil and the Mobil Fluid 424. We finally settled on the 424 and are happy with it after 2,000 miles. But then we were happy with the gear oil as well....

Howard

 

[billiebob]

BUY the oil the manufacturer recimmends PERIOD.....
90% of the oils are identical and there is zero advantage to spending more.

ps, how when overlanding do you operate continuously at high speeds, if anything we operate just above idle

 

[glennm01]

Thanks Howard. I can't help but wonder if what's recommended for the Duonic would be the same for the comparatively plain-jane manual gearbox in an '04. But then, the shop guide indicates SAE 80wt GL-3 for "general" (?) atmospheric temperatures (that's if using gear oil instead of the aforementioned "engine oil/high speed" recommendation). It looks like MF424 is 80wt GL-4, which of course supersedes GL-3, so could be the ticket.

billiebob -- I dig the irony in following a resolute all-caps "PERIOD" -- with several more periods. That aside, I'm not suggesting going against the manufacturer's recommendations, but rather, it's the very thing I'm looking to clarify. You do have a point about high speeds, but it's the highway driving before and after all that "just above idle" driving that concerns me. Slight correction -- the "continuously high speeds" bit is actually how it's worded in the Fuso owner's manual, while the shop guide's exact words are "long period of high-speed driving," which seems a bit more helpful. Screen grab from the shop guide:

 

[Joe917]

The SAE 30 or 40 goes in the engine.
GL-3 SAE80/90 goes in the transmission

 

[glennm01]

Not necessarily. Wider shot: