A lot of the cars i used to service while "teching" as they call it, were often pretty grimey. To the point of effecting my performance, if not just for annoyance reasons.
Purple Power works great and even shines up aluminum nicely. I preferred the engine to be warm, not cold or hot, which really helps to break down grundge.
On our new trucks there shouldn't be any oil leaks. PP is strong biodegradeable stuff and could be heavily diluted for normal "dedusting" duty.
I would never use anything like AmorAll on underhood plastics, unless for show, or one is willing to clean it every week (eventually the plastics will become scratched). Keep the oily residue off and dirt won't build up nearly as much. Again, obvious info here.
I used PP for years, and was always careful where I sprayed with a hose- avoid electrical connections (obviously!). Not once did I cause any problems with this method. Simple Green would probably work well for a newer auto, but not for older cars.
It made my work easier to perform and showed the customers I took pride in what I do; tthe results were "visibly clean"

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Thanks,
Gary

06 Ridgeline RTL S/R, Nighthawk Black, now 23K miles, 20" wheels in the garage; EBC Brake Upgrade; Winter Wear: 17" OE wheels and new Yokohama AT/S 275/60's; S/S nerf bars, S/S bull bar, Hella Fog lights, K&N air and oil filter, Amsoil Synthetic oil. Soon: FIAM air horn; Brake Fluid upgrade.
FS: 1999 BMW M3 Dinan 2dr Coupe, 5spd, only 83K miles, Loaded, $$$$ in upgrades, new tires and brakes, 30+ MPG and 275HP! ONLY $15,850
www.autotrader.com zip code: 97426