Shell Dentax is a straight GL-1 gear oil: "product contains no EP additives". Chances are it will smell like oil, not like EP oil (which stinks). "Normal" rear axle oil is EP.

http://2kwy.net/pds/pdf/Dentax.pdf

There is a Dentax G, and it was SAE 80W90, but I haven't found any reference to Dentax C. If it existed, it may have been a long time ago.

Concerning air filters, it can get expensive to use a dry element in a dusty environment. You use up lots of elements.