D 15 valve seals

I'm kinda thinking there probably aren't any valve seals in that engine . None were used in earlier engines. It sounds to me like the guides might be wore. A valve guide seal will not effect your oil pressure anyway. What oil are you using. I use straight 30w for tractors in the summer, if I had a low pressure situation I might try some heavier oil as long as it's warm. How far down does the pressure go? These old engines never had or needed a lot of pressure. If you have insufficient oil pressure with 30w oil, I would guess the cam, rod and main bearings are probably a little on the loose side.  Charlie

The engine holds a solid 20 lbs of pressure,  dont smoke right away, just when there is enough oil pressure to be oiling the top of the engine, but it does go away. I am using 10 w 40 with a quart a lucas oil conditioner. It runs very good and has plenty of power, but the valve guide issue is very possible.

So if I understand you correctly, you say after the oil pressure builds, then it starts smoking at idle? Quite likely the guides are worn, but you might also want to look to make sure it isn't pooling oil on the head where it could over run the guides and be sucked down them. Had a 460 Ford do that once. Smoked a fair amount on 1 side at idle. (dual exhaust) Rear drain back hole in one head was plugged with a piece of umbrella valve seal. Knocked the piece out and it stopped smoking at idle. My 60 Oliver (has very similar design to B,C,CA engines) that has all new valves and guides on a no expense spared overhaul gives 1 big puff of smoke on a cold start. No smoke running. It has no valve stem seals and never did. My CA will give a puff of smoke on a cold start too. It also got new valves and guides on a complete overhaul.