WD40 isn't nearly as bad for a diesel, though.

Ether is some nasty stuff, and has no business ANYWHERE near a diesel engine. It's an easy way to break a piston ring. And, it's way too easy to get a diesel to runaway on it. Oh, and did I mention... ether lights off so easily, it can explode on the glow plugs, causing major damage, if you use it for the advertised function (as a starting fluid)?

(I've used WD40 myself to start a diesel, FWIW.)