My2cents, don’t confuse compression with compression ratios. The ratio is strictly a calculation, the compression in psi is a function of not only the ratio but also how the engine is aspirated and its inherent pumping losses. The compression actually varies depending on the engine operating conditions. A stock engine operating at low rpms with an almost closed throttle plate and no artificial aspiration (supercharger or turnbocharger) doesn’t develop the amount of pressure that an engine operating at higher speeds with an open throttle plate does, or that an engine with a turbocharger or supercharger pushing air in does.
Compression tests confuse the issue, because they check for the cylinder’s ability to compress gasses strictly based upon the mechanical compression ratio without the engine in operation. They don’t give a reading of the pressures reached during operation.
There are some other variables (N2O, anyone?), but that’s the gist of it.
I agree with OK4450 about the fact that seals do fail over time. They’re just rubber and have a metal shaft spinning in them thousands of time a minute. You can figure for yourself based on the mileage and an estimated average rpm and speed how many times that shaft has turned in the rubber seal… it’ll be a whole bunch. And I’m not even counting the absolute fact that even an unused rubbery bit will deteriorate with age.
I also agree with Busted that it’s crankcase pressure that pushes oil past worn seals, and the Positive Crankcase Ventilation (PCV) system is prudent to check out. If you’re going to change the rear seal, that’s only prudent. Might’s well check everything out.
You need only worry about the compression if it’s too low. That’d mean there’s too much combustion pressure blowing by the rings pressurizing the crankcase. It doesn’t sound based on what you’ve written that that’s the cause of your seal leakage.