I don’t ever wash the engine bay but when I wax and polish twice a year, I wipe down the engine and use meguires natural protectant on all the rubber and plastic surfaces. On the old cars I also used to hit it with clear. No need anymore.
I used to wash my truck’s engine compartment w/a spray wand at the diy car wash. I’d place plastic covering over coil and distributor, but otherwise would spray away. Sometimes it would start up a little balky but quickly return to normal. After I purchased the Corolla and noticed all the electrical connectors in the engine compartment it occurred to me the spray wand approach might not be such a good idea, so I stopped doing it on the truck too. Never noticed any performance difference, clean vs dirty engine compartment. I do shop-vac all the accumulated debris from the engine compartment on every oil change on both truck and Corolla, and think that’s probably a good idea.
I’ve got one of those wands that uses compressed air but then it siphons whatever out of a bottle. It allows a more concentrated spray but the bottle is a hassle so haven’t used it in years. I use gunk on the mower and the garden hose but I’ve gotten a little partial to brake clean now. A little expensive but cleans well and dries quickly.
I use quick detailer (Meguiar’s Qwik Detailer) sprayed on a microfiber cloth to wipe down everything that I can reach. It doesn’t specifically say that it can be used on plastic, but I haven’t seen any problems after ten years.
So far not much, but I’ve only had it 2.5 yrs. It’s an '08 with a little over 45k on it. I know the previous owner (my brother), and really didn’t drive it hardly at all, and he kept up with the basics. I know it will turn into a money pit, but bought it knowing this.So far a couple oil changes, 2 tires, and a sensor for the top.