91' B230F with 150,000 miles has steady oil light lighting up at hot idle conditions. Car is driven mostly around town - short trips under 15 miles
a day.

Recently, I had low oil pressure problem that tested at 4-5 psi
I changed oil and filter, noticing abnormally thin old oil consistency
New oil pressure tested at 30-40 psi at 3-4000rpm
also changed oil sender/sensor and checked wiring harness
thought problem was solved.
However
Oil light, which was previously on all the time, now comes on only
at hot idle conditions.
Then, changed thermostat - old t-stat was defective resulting in engine
only warming up to between 7-8 o'clock on gauge
Changed oil again thinking water was diluting oil, causing low pressure.
Still seeing steady oil pressure light come on at hot idle conditions
Engine runs smoothly. No leaks. No smoke. Coolant level normal.
Suspecting oil dilution. Maybe gas getting into oil? Slight drop in mph
I attributed to cold weather.
Any thoughts? Thanks