Originally Posted by Zack01GC

After being a detailer at a BMW dealership for longer than I'd like to admit, it is fairly safe to rinse a motor. I've never had an issue with mine or any of the hundreds I've done at the dealership, using the same process time and again.

-Cover engine with water-based degreaser/APC (I like Ardex Green Stuff Plus)
-Let sit 5 minutes, but do not let dry
-Rinse with a hose with a nozzle, generously.
-Dry with compressed air via blow gun
-Apply dressing of choice.

Here are the results of that process on my 112k mile 330ci.