Is this oil ok for use in the FD?

Theres also Synth S 5w-40

whats a good viscosity.. sorry for the question but theres not a big range of oils that i can read and understand here in Japan and Mobil is only what the mechanic uses.

 

I use the mobil1 15W50. I wouldn't use something as light as 10W30. The 5W40 is marketed to people who drive european cars to maintain warranty (bmw, VW, Audi, etc). The overwhelming opinion (including mazda's) is that heavier oil should be used with RE's.

 

My owner's manual calls for 10w-30 IIRC.