Yes, I was thinking about a bypass with a relief valve and have it set low (5-10 psi?). I'll probably just keep the throttle down and never have to worry about it again though. That's the easiest solution.Get one with a bypass. You shouldn't have that much pressure on the return line anyways. If you do, there's something wrong then.
The pressure is coming from the high flow rate of the oil through the filter. The filter is a restriction (like back pressure from a muffler). The more flow you try to pass through a restriction the higher the inlet pressure is.
The filter might only be built for 10-15 GPM (no way to determine that as it was a random oil filter from Napa that fit). Try passing 20 GPM oil through a filter that's only rated for 10-15 GPM and it will get ruined.
No worries, the splitter is plenty fast around 300 PTO RPM.