I would say if an oil change really does have anything to do with a PO300, it would be more likely to be internal pressure related. That would make more sense than heat. I guess it could be a little of both though. It's possible though that with too little or too much oil the pressure could jump a little one way or the other and if there's already something quirky causing the code any small variance in internal pressure could throw it back up.
, just a thought.
I've pretty much narrowed mine down to a broken/shorted wire or a possible vaccuum leak. It's going on the oscilloscope tomorrow or thursday, so maybe that'll give me a little more information.