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Both work just fine, just don't do it on a hot motor. The IP really really does not like changing contracting really fast, I forgot what it can trash out, but I know it can.
I just Simple Greened my motor this past weekend. The most important thing is that you make sure the engine is cold. Spray a healthy amount of simple green on there and rinse it off. Your belt will probably squeel for a little while, but it's nothing that some belt dressing won't fix.
I use a cleaning product called "awesome" I get it at dollar general store.
I then use a siphon air gun, stick the suction hose in the awesome and spray the engine compartment, the air helps loosen any build-up of gunk and makes the cleaner penetrate the grime. Let that sit and soak 5-10 mins. but don't let it dry, then stick the suction hose of the siphon air gun in clean water and rinse, if it's not rusty it will look like new. Then I apply a liberal amount of a protector like armorall. You will be very happy with the results.
NOTE: do not let the cleaner get on the fenders, cowl, or windshield. Wet those areas with water before you start, "awesome" will take the wax or protected coating off the painted surface and I have seen it pit a windshield if left to dry on the glass.