The oil pressure relief valve is on the oil pump, so inside the sump.
There are other possible causes but only oil that has thicked or was too thick to start with could be fixed without removing parts.
EDIT:Keep this to one thread please so that people can see what advise has already been offered.
If you read all of the thread
viewtopic.php?f=5&t=198405 then you know over use or in-appropriate use of sealant can block oil ways.
You would also then already know the relief valve is on the oil pump.