I should elaborate. The common "blown seal" diagnosis is mostly a myth or mistunderstanding at best. Top image above shows a GTX ball bearing cutaway; I borrowed the pic from some site but it looks like it was taken at SEMA. Bottom diagram is an old one but still mostly applicable.
The seal systems on both sides are dynamic, meaning they are not effective unless the turbo is spinning and everything is up to temp/pressure. At the core of each seal is a piston ring, but this is only designed to restrict gas flow - not oil flow - into and out of the turbo. The idea with oil is to keep it away from the piston rings. Pretty simple strategy really..."How do you avoid an oil leak? Keep oil away from the holes..."
The slinger grooves on either side act to spray oil away from the piston ring bore area and into the big annular (mmm, donut) cavity around the shaft on each side. Sometimes there are oil deflectors as well, but many turbos don't use them. On a ball bearing turbo you do get some spray out of each open end of the bearing cartridge which heads right towards the seals. This is why it's more important to restrict oil inlet flow with a BB turbo. Oil loses pressure as soon as it exits the bearing, so the drain cavity in the center housing should be mostly air ideally, with oil vapor and foam draining down the chute and into the drain hose or line.
Back to the idea of piston rings acting to seal gas pressure: if crankcase pressure is too high and the drain's connected directly to the crankcase, then you'll get essentially the same pressure in the turbo center housing cavity. It acts to force oil past the seals on both ends, depending on conditions on the other side of the seal (so leakage is worst at low boost / low backpressure conditions like idle or overrun).
In Tom's current situation I think crankcase pressure isn't the problem, because the drain goes down into liquid oil, isolating the turbo from crankcase pressure. Gotta be insufficient volume to drain the center housing. In the case of the mostly-horizontal drain line heading back into the block above oil level, it's likely still too restrictive - AND also now directly feeding crankcase pressure into the turbo, where there's too much oil hanging around waiting to get blown out past the piston rings.