frequency of oil changes

[Mark McDonald <tomswift@xxxxxxxxxxxxx>]

I changed the oil in my '68 yesterday and got to wondering.  In the 
FSM, it says to change the oil every 3 months or 4000 miles, whichever 
comes first.  I haven't put on 4000 miles on this car since I bought 
it, so I haven't changed the oil in 3 years, figuring, well, mileage is 
the key thing.  But now I'm wondering if time itself is a factor.  Will 
old oil go "bad" in the crankcase?  Should you change your oil every 3 
months REGARDLESS of how much you drive?

Thanks, Mark