DRHP has it right.
I ran into the PCV problem right away when I put the blower on late last fall. We put vacuum gages on both the manifold and under the carb. I don't remember the numbers as it's been a few months but there was a huge difference between NA and blown. As was noted there is always vacuum under the carb with the blower. I have 2 750 DP on a 355. I already had tall valve covers with baffles and still pulled oil into the PCV. I added a secondary baffle which seems to have cured it. I only run the PCV to the rear carb. Coasting down from a charge down the highway I can get 21 inches of vac. My 89 T-Bird Super Coupe used to get even more vac. It ran over 200k miles with very few problems. 100k on a set of plugs. Possibly a PCV from one of these might be a good choice. I'm going to check it out.
We just took the motor out and lifted the blower off. There was no oil in the manifold, in fact it was very clean. The plugs however were pretty black but they have been in since I first started this motor with the RPM manifold and single vac sec 750.
The few times I checked the plugs while running the blower after making a few hard passes on the road they seem to clean up pretty well but get black quickly just idling around. I run pretty rich at cruise 11.8 to 12.2 AFR at the last of the year. If you really jump on it at 2000 rpm or less it will puff some black smoke. Sometimes it's hard to tell with the tire smoke. haha
I was cleaning things up yesterday and noted that there was no oil in modulator line or the power brake line. Both are quite long and I hung them over a clean paper towel. so far nothing is dripping out.
Got a new pulley from the kids for Christmas so I'll be moving on up to "big boy" class now haha.
Just a thought, It's possible that the blower front cover seals have gone away and you are pulling gear oil in. We used to run Bakers AAA Castor oil in the blowers on the race cars. My son says they still do. You can sure tell if there is a leak as there is a very unique smell to burning Castor oil. You may have a pressure relief valve in the blower front cover near the top of the case. You should not relieve the pressure unless you are storing the blower for a long time. These lip seals keep the oil in and if you release the pressure they can leak. I took the valve out and installed a pipe plug before I ever put the blower on. My blower has been sitting for 6 weeks now and there is not a drop of oil inside of it.