You're probably getting a combustion leak into the cooling system, causing the pressure to increase greatly. Could be anything from a head gasket to a cracked head/block.
Check it, or get it checked with one of these:
Block check If it's positive, you need to pull the head (pretty easy on one of those) and get it checked for cracks. Then take it from there.
If it's negative then you most likely aren't circulating coolant correctly; bad/wrong stat, bad/wrong pump, incorrect fill procedure, or more.
Hot oil will be thin. Water will turn it milky.
