Pilot reported a loss of oil pressure and rise in oil temperature and began a descent to warmer air. The Pilot lost communications with Center and was unable to report their IFR cancellation in a timely manner.

Narrative

While I'm cruise flight at 12;000 ft.the aircraft oil temp began to rise with a corresponding decrease in oil pressure. This was a re occurrence of an earlier issue perhaps related to a temperature probe freezing which rectified during descent and warmer OAT. I [requested priority handling] and began a descent while communicating with ZZZ Center. Communications were coming in broken with ATC due to terrain but I advised I had ZZZ in sight. I later advised engine was back to normal with the descent to warmer air. ATC advised to switch to advisory and I said I would cancel IFR and continue towards ZZZ as the engine was back to normal. I believed ATC said squawk VFR but communications were very broken at this point. I switched to ZZZ Unicom and continued south with the aircraft performing normally. I thought I squawked VFR by hitting the VFR button but 5 minutes later saw I was still on a code and must have missed the button with turbulence. I squawked VFR and was monitoring 121.5 but heard nothing. I later was contacted by ZZZ1 Tower and then ZZZ1 Center. I advised I had cancelled IFR and they advised ZZZ Center hadn't heard. I telephoned ZZZ Center and advised I had a whole radio conversation they hadn't heard. We discussed making sure they respond to the IFR cancellation because emergency services was notified.