coolant and oil mixed dripping at bottom of radiator

[JimT]

first things first, find the location where coolant is leaking and fix that! 1 gallon in 150 miles is serious and could get much worse at any minute.

As tree98 said, the oil could be coming from somewhere else, such as the breather. If you're over full on oil I'd check your crankcase breather filter and see if it's full of oil. Drill it out or replace with a maintenance free filter. You don't want it getting clogged up with oil then not venting pressure from the crankcase. Being a little over full shouldn't hurt. make sure you check it cold on level ground. I know mine can vary in reading up to 2 quarts depending on the angle of the engine and how long it's been sitting.

If you're not seeing oil floating on top of the coolant reservoir then it's a good chance that the oil is coming from some other location and mixing externally.