Gary,
The oil pipe work inside the D oil tank should be similar to the C UFM in concept:
If you peer into the oil filler hole you should see the oil block that is brazed in and it should have a screw in the top for adjusting the oil flow to the chain oiler. This is where you problem lies. When your engine is running there is a small hole in the side of that block. The block being attached to the oil return line, the oil coming out of the block is your true oil return oil stream.
Unfortunately, the oil return stream of oil, being so high up in the tank and so near your vented oil cap I suspect it is coming out of the oil vent hole or around the old oil cap gasket, or both. This is not unusual. On the Series C bikes it was not unusual to find when racing your Vin that putting your chin near the tank as you headed down the straight which is good for stream lining, would get you a squirt of oil on your face shield.
I originally made a disc from a beer can with a hole near the edge that I could install under the screw in the block. This worked fine until someone helping me in the pits dropped the disc into the tank. I simply made another, but months later when I removed the UFM I made an "idiot proof" one shaped like a carb slide and attached to the cap. This was perfect. Unfortunately, again, when I went to make another I realized that these brazed in blocks were not uniform in their brazed in positions and I found there was little room to do it on the current race bike. The cap was quite close to the block.
This slide is just diverting the oil away from the oil cap. You don't want whatever diverter you come up with to be air tight as well because the cap needs to vent the air in the tank.
Check the gasket and you may want to try your skills with the beer can for a test...of your mechanical skills, that is.
David