Oil return line

Yes . Set it at 87 lbs 3 were 85+ and one was 81Lbs

So 3 cylinders had less than a 2psi differential and the other one was out by 6 psi.  >3x the others.

That is a suspicious reading and maybe what is causing your high crankcase pressure.
This is definitely worth investigating.

I am afraid I have run out things to investigate . The scope looks good no ware . Even 6 lbs is well within limits .  If that was the case the motor would need to be rebuilt every other year as the pressures get farther apart it would really increase pressures?. That makes no sense.

I have consulted another well respected builder of the big bore kits and said they are producing higher case pressures. He mentioned why does the 915  have 2 return lines and a max pressure of 5.8. if they are not hving issues?

I have asked the builder of this engine to test his other similar engines to see what the pressures are running but he won't seem to try that.

If I have to tear this 100 hour engine down It will cost more than it's worth and still no guarantee we will find anything wrong as it runs fine with good compressions . It just blows main seals about every 40 hours.

With The new larger return lines it seems to stay under 9 lbs at full power with the  oil at 225 and 5150 rpm. the question is what will happen at cruise 5500 on a hot day?

Is there any problem running a second return line?

Your Original Post mentioned...
I have been chasing oil leaks and blown main seals on a freshly overhauled and Modified 912 …

The latest post Mentioned ...
I have consulted another well-respected builder of the Big Bore kits …

Are we talking about a 912ULS with Stock Cylinders, or something else?
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If properly executed, Dual return lines should not cause any major issue.
You may need to obtain a 915is Oil tank with dual return ports.

It is an edge performance big bore 1625 I think on an old 1999 engine with old style fuel injection.