10W-30 or 5W-30 or 40 would be just fine....Think of your mouth as the top end of your engine.....and 20W is a milkshake and 5W is water. suck through a thin straw and what will get to your mouth (top end) way faster.
MOB - your in a warmer climate than I am (Michigan) so you might not need such a thin oil. Just use the guidline for PSI at warm idle listed above.
I think my old vert would be aroudn 25PSI on 10w-40 in the summer and a little less in the winter. But in the winter, I didn't (will not) drive around in the Fox so it just ran in the garage to keep things working properly.
Pressure is a resistance to flow. The more pressure, the more resistance. The more resistance...
P.S. listen to your engine builder (if you have one) he is above me in the chain of comand.
I used to run 10-40 in my old stang... new one I run 15-50 mobil 1. Guy who had the motor built ran 20-50. At least according to the dummy oil pressure guage I get 3/4 98% of the time, the other 2% it drops just below the 3/4 line after cruising for a over an hour or so then coming to a quick stoplight or something.
I too am an autozone parts monkey (parts supervisor monkey)... when customers ask... I ask how many miles. If it's over 75-100... I just say "I'd use 10w-30 or 10w-40... it's up to you" other than that... dunno- go with the factory or get something thicker- it's up to them
The statement about 20 weight being what it is in winter is interesting. SAE weights are established at standardized temps (0 and 100*C, as I recall). That's the basis for the viscosity ratings, plain and simple.
The w in 20w stands for winter and I only ment that on the inatial startup for the first few seconds until the motor starts to warm. I Made a mistake I ment 15w 40 when I was talking about the rotilla oil not 20w 50. I agee about blanket oil statements and if it sounded Like I was making one I was not trying to. Here is a link to a really good article it is mainly geared to flat tappets but stands the same for roller cams.-http://www.hotrod.com/techarticles/engine/flat_tappet_cam_tech/
If you read the charts it talks about the zddp content which is the main friction modifer in conventional oils and Like a I said before about the newer conventional oils becoming cheap quality these additive are disapering hence major bearing and cam lobe wear.