Just got a new to me D3B and it's got an oil leak, some from the valve cover gasket, some seemingly from the gasket underneath that. And also the breather boot is cracked so some is shooting out of that as well. Needless to say the whole motor is covered in oil, I took the side panels off I'm wondering if it's okay to directly power wash the motor? I was going to spray it with some dawn or simple green and give it a light pressure wash to hopefully get it decently clean. If not okay to power wash lmk any suggestions for degreasing it.
I think if you're careful and don't overdue it a light pressure wash like you say should be OK. Don't spray around any electrical connections or where water could get into the air filter. I had a fuel filter with a pin hole leak spray diesel all over the back of the engine and firewall in my 931B which is the loader version of a D3B. Use degreaser and basically just rinse it off with not too much pressure.
Put a plastic bag over the alternator and like Dave said don't get the air filter wet and have at it, I can't imagine a good pressure wash would harm anything other than those mentioned here.
Don't use the smallest most aggressive nozzle and spray with the wand back aways from the engine.