I have noticed in the past few months that as my truck warms up my oil pressure drops while I am at idle. Normal running temp is about 175 to 185. But since it has been warming up around here during the day I get up to about 190 to 200 near the coast and if I go to the high desert it gets to about 210 to 215.

Until it warms up my pressures are right around 40 PSI and stays there till it stays warm for a bit, after it warms up it drops down to near the lower white line, as soon as I hit the gas it goes back up to normal.

I recently changed the oil using 10-w30 Castrol High Millage oil, but could not do the filter due to douche bags at walmart. I bought a Mopar filter and when I got back home and went to go put it on, out popped a fram filter that did not fit.

 

This is actually quite normal. The multi-weight oils do this as the heat and cool. When you startup from a cold engine, the oil is quite thick and thus will have more pressure flowing through your engine. When it heats, it gets quite a bit thinner and will flow more freely. This is called viscosity. As long as it doesn't get too low, you're fine.

 

It doesn't have to be multi weight , any oil will thin out when it's hot.

 

Yeah, I think I was confused. The multi weight oils try to minimize viscosity change.. right?

 

Basically they have the pour point and pump-ability of a thin oil and film strength and protection of a heavy oil.
That's my understanding of it, not a very technical description but my own words.