It's certainly not good for the health of the cooling system to have old coolant in it - this becomes acidic over time and can corrode away things such as freeze plugs, aluminum intakes, water pump bearings, and yes - cylinder head gaskets.
Oil on the inside of the cap is NEVER normal. Nor is any sort of foam - so this lends credence to the head gasket theory.
FWIW the compression on my 320k-something 350 was around the 90-95PSI mark on all cylinders, which is what led to my head replacement about a month ago (at the same time battling strep throat and a complication of same called 'acute post-streptococcal glomerulonephritis'). The car went from 12-15MPG to 15-24MPG as a direct result of the head swap.
__________________
1988 9C1 - Modified LM1 @ 275HP/350TQ - TH700R4 - 3.08 8.5" Disc Rear - see it at
http://www.silicon212.org/9c1!
2005 Crown Vic P71 - former AZ DPS - 4.6 liters of pure creamy slothness!
1967 El Camino L79/M20 old school asphalt raper
Remember - a government that is strong enough to give you everything you need, is also strong enough to take everything you have.