Lately I have been experiencing erratic oil pressure in my 912. Upon startup I get instant oil pressure as normal. After warm up and takeoff I fly for awhile and sometimes my oil pressure dips down to 20 briefly and goes back up to 40. This may happen several times but usually not when the engine is good and hot say 200f degrees. I'm using 20W50 Honda oil. I know that's a bit thick for winter but it averages 60f degrees around here. Any ideas?