3 turbos, same problem.. has to be something other than the turbos. My first guess would be the oil drain from the turbo, blocked or not draining properly. Its gonna take the easiest path out and thats around the bearings..
guy i know used to build turbo kits always had extra big drain lines off the turbo and made sure that the point that the oild returned into the engine had plenty of clearance. the oil gets all bubbled up and hot as it passes thru the bearing and needs all the help it can get getting out of there and back into the main oil system.