i have dark oil always, and it leaks slowly from perdy much every seal in the engine, i wouldnt jump straight into head gasket problems, but it is a possibility,
to check for head gasket problems look at your coolant and see if it empties itself, you can check for oil residue in the coolant aswel, and also check ur oil dipstick and see if you can see any white bubbles (coolant)
could just be that your seals have begun to leak (Im always an optimist
)
also what is 40 odd degrees out????
good luck
Jake
P.S. I drove my liberty round with a blown head gasket for six months, just removed the thermostat
very very dodgy but it got me out of trouble till i had some money