Sounds to me like a bad head gasket leak. Usually when you have a right side crank seal leak it will spew oil out the right pipe, It will not be describable as almost transparent, it will be dark. If you have a crank seal leak on the left side, it will such air and lean out. That plug would look ashy, possibly have metal specs all over it depending on how bad the air leak is and how long you ran it that way. I would also, as mentioned above, do a compression test. It may show an obvious problem in which case you will have to tear it down anyway. If your compression test looks good, I would tear the head off and take a look at the gasket. You will need to replace it, so have a spare(unless you run a cool head or O-ringd stock head). If you have the means, a leakdown test would also be a good idea, and would indicate if you have a bum seal or air leak of any sort.