RAM switched from a seperate oil tank to the fuel/oil mix in mid-1999 with the introduction of the "F" engine.
By "Superman" in 1999, the "F" engine were on the street in large numbers.

Rei and Albert (owners of RAM) supplied fuel to all of their customers at the Superman meet.

My first RAM was an early oil tank engine, from the first production run...S/N 0043. The early engines like mine had a black engine casing.

As I recall, the "F" engines introduced the gray colored casing, and that color continued until the end of RAM production.