If he is saying seeping oil then a sealant won't help that. If the valve cover is the cause of the oil seep then that is a different story and isn't a real costly issue. I used a sealant pellet I actually purchased at a ford dealer that stopped a timing cover leak on one of my 2001 Taurus for a few years. At least the package stated used by OEM on all new engines. I went back a few years later and they said they don't carry them anymore.
I had a 1972 Dodge Colt that had an oil leak in the back of the head. Finally it was gushing oil and I removed the head only to find the copper ring that sealed the block to head port was damaged. Actually the mechanic that replaced the head gasket years before cut it because it was not in the right position. It held for years but finally leaked. If the oil leak is being caused by by the sealing between the head and block it may get worse fast.
I had a 1972 Dodge Colt that had an oil leak in the back of the head. Finally it was gushing oil and I removed the head only to find the copper ring that sealed the block to head port was damaged. Actually the mechanic that replaced the head gasket years before cut it because it was not in the right position. It held for years but finally leaked. If the oil leak is being caused by by the sealing between the head and block it may get worse fast.