James,
It sounds like the engine might have quite a few miles on it. Although not at the top of the pressure range, I think it is adequate to lubricate the engine properly.
You might want to change the oil, and when doing so, consider a multi-grade, or at least one step up in viscosity.
Make sure you don't have a fuel dilution problem from a leaking fuel pump... this could be indicated by a overfilled crankcase, oil that smells of fuel or oil that appears very thin in viscosity. You have the right idea...change it and check it!
Stuart
