Absolutely Bob, oil viscosity is critical in the system flow characteristics and thus pressure readings.

Dave I agree totally with Bob on the viscosity. And like he says it does depend on the temperature. Synthetics tend to have a higher viscosity index. The viscosity index gives you a quick reading on how well the oil viscosity holds up versus temperature.

The higher viscosity oils will provide more hydrodynamic action than the lower grades but like Bob said at the price of higher friction and in the end higher temperature rises. There's no free lunch!

BTW Chevy says the warranty is void on my 502 if I use anything but 5W-30! Since the engine has been in my garage for 2 years now that is a mute point . But as you probably know this is an engine designed originally for marine power boat applications cruising at 3500 rpm all day long! I plan on using a synthetic 5W-30 which will actually outperform a fossil based 10W-40. And monitor the oil temperature ..........

In your case I say stick with what you know works!

Regards, Kitz