a thick, straight weight oil like the 50 weight W100 is going to produce a lot more pressure when cold due to it's viscosity when cold. This is normal and expected. It's too thick for winter op's in my opinion (read the recommend temp ranges in the Operators manual). If you have properly confirmed the gauge, you have some work to do. I disagree with the other poster that this has damaged your engine. However, it does need to be addressed, lest you end up with no pressure in the air some day and get a forced landing with major engine damage.
Ed covered this pretty well. 1) failing pump 2) problems with pressure relief system bleeding off too much oil (debris in the seat is somewhat common and produces these symptoms) 3) excessive downstream leakage (bad bearings, leak, etc.) 4) pump inlet blockage .
Bypasses are in place to keep blocked coolers and filters from causing this problem.
The fact that it showed up- quickly points toward problems in the pressure relief or pump inlet blockage, though all are possible and in play. Lycoming pumps have had a history of self-destructing in the past though I think the current generation are fairly reliable.
EDIT: I would also ask the builder / previous owner if it always ran at 35 PSI warm, in cruise. If this is the case and the OP instrument has been properly verified, I would be looking closely for a manufacturer defect in the pressure relief assembly, as well as confirming that the correct bearings were installed in the engine during it's overhaul. For example, if the crank was ground M003 and STD bearings were used, you would see this type of problem. In theory, you would catch this when checking main bearing clearances, but I have a hunch that this check is not routinely on lycoming rebuilds, due to the difficulty in case splitting.
RV-6A / IO-320, Flying as of 8/2015
RV-10 in progress
Last edited by lr172 : 12-08-2018 at 09:24 AM.