360 oiling issue

Hello all. Working on a rebuilt 360 that I put in a 72 Ply Val. used Royal purple brake in oil, and all seemed to do well. Used cheep oil filters, as I changed them often during the run in (Plan on useing Mopar filters after brake in). Car has sat for 2 weeks as I get everything together to put on the headers and new TTi system. Last night I pulled the oil filter off to test fit the pass side header, and found that there was very little oil in the filter. Sat it on the bench and took a sample of oil to send off for partical test. Installed the header and went to the bench. and noticed the filter was about half full. Now, I am useing the 90deg filter mount so I dont think the oil siphoned back. But could it. I will put a pressure guage on it this weekend and restart and check it out. Any Ideas. I dont see any blockage, and the rockers get so much oil that it will shoot out over the fenders when priming with a drill.