What's wrong with a too-big cap?
Yes, in tone filters, you need a right-size cap.
But in power filters, bypassing, and coupling, usually you just need an "at-least" cap. A 100 ohm load needs a 100uFd cap to be less than 3dB down at 20Hz, but a 200uFd cap will also be less than 3dB down at 20Hz, so that's cool too. 40uFd may reduce power ripple enough to meet the spec, but 60uFd will also meet spec (better).
So a lot of caps are sold as "minimum" or "-20%/+50%", very loose tolerance where you often get "more than you asked for". It is quicker to churn out 11, 12, 13uFd caps than to sit there and trim each one to 10.0uFd.
Capacitance won't rise with age. (No free lunch, even if you wait.)
Don't confuse capacitance with leakage. Old electros go leaky (I don't think oil-caps do). A really dumb capacity meter might not know the difference between capacitance and leakage; if yours doesn't, you shouldn't be buying old caps.
Check leakage with a 9V battery, cap, and 100K resistor in series. Put a voltmeter across the 100K, and wait about an hour. The voltage will drop from 9V to 0V. For oil caps it should drop to a few millivolts after an hour. You might want to try again with the maximum voltage the cap will see in service, to be sure the oil has not got damp and lost breakdown voltage. But that is dangerous, since oil caps are usually used in high voltage systems. (No reason they can't be used at low voltage.)
