I have a 66 coupe, 200 ci motor, 3 spd manual tranny,,,,lately the oil pressure guage has been sitting at a hair lower than the half way mark and as the motor warms up the guage starts sinking to about the 1/4 mark ( hot as all get out in Phoenix arizona),, I understand oil thins out as it warms up, but I'm using 10-40 valvoline synthetic oil, needless to say if if drops to far it's a disaster,, question is is how far is to low? should I switch to 20-50 weight,,, the low end of the motor has never been rebuilt and I'm thinking oil pump replacement,, no knocking or rattling yet,,, any input would be appreciated

 

You may want to try a mech gauge to see what the pressure actually is, and its not a gauge or sender issue. I like synthetic oil in an older engine. It has been my past experience though to see a slight drop in pressure when switching from dyno to synthetic. Even with the same viscosity. I'm not sure why.

 

Either your pump is bad or as evil sad, your gauge is bad.

 

or some several components are not fitting as tight as the use (worn out) and thus causing a drop in oil pressure.

+1 on switching to a mech gauge.

-Gun

 

Drive a Chevy. Their oil pressure is all over the place.

I wouldn't worry about it. It's an old motor that's loosened up.
As long as you have some pressure at idle, and more when you step on the gas you're okay. The general rule of thumb is 10psi for every thousand RPM.

Worry if you have NO pressure.

 

Hey now, my truck's oil pressure is steady and it's a 76 Chevy.