its not a rear main seal and probably not a head gasket. if u take off the oil cap and theres a foamy, milkshake lookin shit in there then its a head gasket and if theres water in ur oil. in all seriousness i highly doubt it unless u have been overheating and all but u could just check that stuff and do comression test to tell. also do a leakdown test. and since ur doing that, if u hear the air coming through the oil then its probably rings. how many miles on the motor? does it do it when the engine is under high deceleration? and nobody can tell u what to do bc it depends on what u wanna do with the car weather u build the shit out of a motor or just put a stock one in