Piston blowby is the most usual culprit...i had this problem and was spitting oil out everywere.
Chad
Oil leak starting to scare me
I had one recently blowing oil out of the tranny vent. I tore it down to put a crank in it and found that the bracket that transfers oil to the bridge and holds the timing chain tensioner shoe was loose. There is an 0-ring underneath it and it was letting pressurized engine oil escape into the transmission. The result was an oil starved head with worn out cams, buckets etc...$$$$$