We had a customer experience a similar situation on a 1968 Triumph 650 recently. Upon start-up, everything was fine until about 20-30 seconds after running. Oil started to belch out of the breather tube. An immediate check of crankcase oil contents revealed about 5 or 6 ounces drained from the bottom end. 100cc was then replenished through the timing hole aperture. The engine was started again, and sure enough, excessive oil blowing through the breather outlet hose again. The curious thing was that there was some oil returning to the oil tank, although a lower volume. After a lot of head-scratching and cussing, it was revealed the customer had replaced the original sump filter/drain plug with a shiny aftermarket plug complete with a magnetic insert. The scavenging pick-up tube was very close to the magnetic insert, effectively reducing the oil evacuation, and causing the crankcase to fill with oil. This was proven when the customer reverted back to the original filter/drain plug assembly.
-Dave