Every engine is a little different, so the best benchmark for comparison is your own engine. If you now have 10-15 less psi under the same conditions, then something has changed. You might send your oil out for analysis on the next change (Blackstone Laboratories | Oil Analysis - Learn What's Going On In Your Engine) to see if excessive bearing metals are present. Otherwise, it's pretty reasonable that after 20 years, your engine would benefit from a teardown and refresh. On the other hand, you have plenty of pressure at operating speeds, so another option is to just keep driving it and see what happens.
