10W-30, just like I put in my old CJ-7.

Only half kidding, I do seriously use that or vegetable oil when honing on the Arkansas stones. Only because I'm a mechanic, and it doesn't cost money to save the half-quart bottle left over after an oil change.

If I were planning to use it as a general protectant for a razor that isn't in my everyday use rotation, then Ballistol would be my go-to. Good enough for the steel in my Weatherby hunting rifles, more than good enough for the steel on my straight razors.