Is oil weight based on oil pressure?

[1974dodgecharger]

Was reading a old hot rod article and they were talking about oil pressure on old mopars......If you have good idle oil pressure at 10w-30 (30psi and above) there is no reason to use 20w-50 as most people do.

Would this hold true?  I switch between 10w-30 and 20w-50 for summers...I always thought oil weights were based on temps. based? 

When I used 20w-50 in the hot summer I idled at 50psi and and with 10w-30 I idle at 40psi in hot Arizona summers.

[Ghoste]

Yes and no and it depends.  Oil "weight" is a measurement of viscosity or its "thickness" at a predetermined temperature so in a hot Arizona summer a heavier oil can well be needed.  It also depends on what an individual engine requires and certainly one with a higher viscosity will pump at a higher pressure since it resists flow more.
40 and 50 psi are both lots of idle pressure assuming your bearing clearances are happy with it so my very very unqualified opinion would be that you are fine doing what you're doing but you may not be needing the 50 weight.  (what is the idle pressure in the summer if you use the other?  what is the oil pressure at high rpm?)

[1974dodgecharger]

at cruise the 20w-50 weight is like 60psi and cruise for the 10w-30 is 55psi......

[ottawamerc]

From what I understand too is that if your using a higher viscosity oil than needed your actually robbing some horse power having to pump the thicker oil

Scott

[Ghoste]

Yep, is it enough to be concerned about in a street car?  Maybe not so much.  Depends again I guess.

[c00nhunterjoe]

If its at 55 psi then there is no need to go higher. The stock pump is probably bypassing with the 20w50 in it at 60 psi. The weight required is based more on your build and how tight it is. 20w50 in my car is bad. Cold start it will bury a 100 psi gauge. Thats territory for exploding oil filters. Even hot on the 20w50 is 70-80 psi. I run 10w30 or 10w40 with 50-60 cold and 35-45 hot idle/ cruise