If its the head gasket you would probably see more symptoms (like the ones you mention, mushy oil etc and hotter running temps) If you haven't seen these I'll wager its a leaky cooling system. Can you smell the coolant when popping the bonnet?

The standard header tanks leak like a sieve after they have perished through age with normal weak points being about half way up on the front and back (back is normally worse than the front for some reason!) Failing that check all the hoses for leaks.

Good luck (hope its something minor!)