Ok thanks Herc. He hasn't done any off-roading recently but when he bought it, it did have a history of off-roading.
I forgot to add that he is losing about a quart of oil every 1-2 weeks now.
I did a little researching and it seems as though the PCV valve could cause the issue too. Another idea was this (from some website):
"The "slime" is an oil/water emulsion, often similar to mayonnaise in appearance and consistency. This will sometimes form in perfectly normal engines under certain operating conditions (in particular, short trips in cold weather, leading to condensation). Now that the temperatures are coming up a bit, you could clean up the oil cap and pipe and see if the emulsion returns."
...which could be true
But I guess worst case it could be blown head gasket, cracked head or cracked block.