I know this is a third topic on essentially the same subject but I find that once the initial question is answered in one thread folks often tend to ignore additional questions in the thread.

I am using 10w40 in an old somewhat tired slant six (no telling how many miles are on it). The oil pressere at hot idle is somewhere between 5-10 PSI, and at highway speed I get a good 35-40 PSI so the pump is functioning. This would all be fine according to the 10 psi for every 1000RPM. My question is, is more oil pressure better? The guy at the auto parts store said that I should switch to SAE30, and that my pressure will come up a lot. I live in the PNW where average temperature arnges between about 20F for extreme low and 90f for extremem high. Is this SAE30 good for such a climate?

multigrade oil is oversold. keep in mind that when slants where designed there was no thing such as multigrade oil

sae 30 to sae 40 (depending on the climate and other factors) where the factory's recommendation.

If you do lots of city driving use a 20w50 oil since it has detergents that would prevent your engine to build up gunk and residues

My '62 Lancer's instruction manual mentions 10W30, 20W40 and 20W20 oils, as well as SAE30. And my July 1964 Chrysler Canada Industrial Slant-6 Parts and Service manual recommends 20W40, 10W30, 10W20, and 5W20 in addition to SAE30, SAE20W, SAE10W and SAE5W, depending on ambient temperatures.
It's entirely possible that multigrade oils didn't appear in constantly-warm climates like yours for many years after they appeared up here, because...well, when the weather only varies from 20°C to 40°C, there's really no need! Up here we have ranges from -30°C to 40°C, so it's natural that multigrade oils would appear in this market first.
All mass-market engine oil, regardless of viscosity grade, contains detergents. Nondetergent engine oil can still be found if you look hard -- it's good for lubricating chainsaw chains, but that's about it. The viscosity grade should be chosen according to the ambient temperatures and the condition of the engine. 20W50 is overly thick for an engine in good condition in most any commonly-encountered temperature range. It is a mistaken notion that heavier oil offers more or better protection to an engine in good condition than lighter oil. All heavier oil does is take longer to get to lubrication-critical parts on cold start, and sap more engine power due to greater pumping losses. On a worn engine with loose clearances, of course, heavier oil can help maintain acceptable oil pressure. Pretty much everyone reading this who has an engine in good condition should be running 10W30.

Avoid synthetic on older seals. I learned that the hard way.