Best homemade penetrating oil, gun scrubber, etc. is 50/50 ATF and Acetone. Caution, this may damage some stock finishes and/or plastics. Mix it up and store it in the can the Acetone (hardware store will have it and you need some ATF around for the car anyway, right?) came in, NOT glass! And do relabel the can.
This stuff REALLY works and is cheap.
BTW, I have and use Kroil, as well as a number of other products, but when they are gone I will not re-supply, the above will do it.