I use Simple Green for everything (almost )

The only thing about it is that it's slightly acid, so you have to rinse very thoroughly. As long as you do that, it's harmless.

What I actually want to try next for waterless drivetrain cleaning is CitraSolv. It claims to contain no petroleum distillates and no toxic solvents (unlike most of the citrus degreasers on the market). Anyone use that? TriFlow is seriously toxic.