oil pressure fluctuating.

more than 15 PSI HOT @ Idle is fine... Highwayspeed.... 45-50 PSI is also fine.

The rule of thump is 10 PSI every 1000 RPM so you are in the save range.

Concerning the weight... 10 w40 would be my favorite...10W30...is "on the edge"...if you are running your engine HARD....the "30" could probably lead to a problem... ( surely on a very hot day )

Greetings Juergen