From what I've heard, it's fine, excpet you need to cover the distributor and electrical wiring or something like that, check with someone before you try it as I am not really sure what to cover. I like a clean engine bay, but I wouldn't do it.
dont do it... some people spray their engine bays, bu its not a good idea... besides, the your is300 cant be THAT old... so u should just wipe off the parts in engine bay... make sure the engine`s cool to the touch and use a wet cloth or rag... it helps to have some kind of non-waxy cleaner on it, like windex or engine degreaser.... when using engine degreaser read the instructions and use as directed... windex worked fine for me and doesnt stain...
mmm I think simple green will leave a little bit of a film. I have sprayed mine of but with a low pressure and did it wile the engine is running. just stay away from the wire box on the drivers side. also you can use some kind of car show sprays that have teflon in it that you can wipe on it works nice.
Simple green works great for me. Like WHITE IS300 said dillute it a bit, also just use low pressure from the hose to wash everything away. It perfectly fine to get the engine wet just dont use too much pressure espically around the electrical connectors and such. I was y engine bay down about once and month. I 've done this with all of my cars in the past and NEVER had a problem.
I don't have an IS300, so I don't know whether these parts are already covered or not, but this is how I clean my engine bay
Cover distributor, alternator, and battery terminals with saran wrap or plastic bags. Spray off engine and hood liberally with high pressure hose, until all loose dirt and oil comes off. After I spray Gunk citrus engine degreaser everywhere, let it sit for 5 minutes, and I brush all the parts that need it. I let it sit again, and I spray it off for 5 minutes after. With the hood open, start the engine and let it idle for 15 minutes. Let it cool down for a couple of hours, and spray a whole can of engine finish. It's a clearcoat for your engine, it will shine up your hoses and it will give metal parts a shiny deep wet looks.