Hi
My husband is a mechanic with his own workshop and his hands are as tough as old boots and so are all the staff, even the apprentices!! They get used to touching hot items etc and it is amazing to see.
However, he uses a scrub on his hands that he pays a fortune for from BMW here in Australia. It is really like a big tub of salt scrub with lots of sand in it and it smells like field of orange blossom! It is really really gritty and the consistency of mashed potato!
Also although turpentine is great at removing grease most mechanics dont use it anymore as they always seem to have cuts or burns from the engine on their hands and the turpentine stings - a lot! Plus there are better ingredients now that dont have the hazardous side to them that Anne-Marie has pointed out.
So far jojoba spheres, pumice and loofah have worked really well for Joe and his staff and yes I have tested most things on them - they are my guinea pigs for nearly all my soaps - if it affects them it is a definite no no

The more scrubbie things you can get into the soap the better but bear in mind they do get burns from hot engines.
Hope this helps you out.
Jo