Oil return line

The 6.53 (0.45 bar) number comes from Second Edition (Aug 2012) Installation Manual Section 79-00-00 page 6.
The current Third Edition (Jan 2021) Installation Manual has now raised that number to a more realistic 0.6 bar (8.7 psi).

The Service Instruction from Feb 2000 has an obvious typo as the 0.3 bar (4.35 psi) number is the same as the number spec'd for the Oil Suction line in the paragraph above.

8.7 PSI is the current number!

"Read and Understand"  is not the same thing as Blindly following Instructions. 
Always be willing to question the instructions.

I just wanna be able to trust this engine that wont blow the seals out. When it's hot full rpm on the ground it jumps to 8.7. So before i went to a larger return line it may have been much higher.

We are looking adding a second oil return line like they did with the 915.

Did you ever see if loosening the Oil Tank Cap had any effect?
If the Tank Vent Line is over a Meter long it might need to be enlarged.

Bill

Yeah we checked the cap and the vent line.  Made no difference.  We even pumped 6 lbs of air psi into the case it vented fine. Still peaks out at just under 9 lbs. More leak down testing next then adding a second return line like the 915?

Just to cover all bases; have you done a cylinder compression test?
If there is a compression leak into the crankcase, it will be receiving an excess amount of blowby gasses that will raise the crankcase pressure.