Oil pressure trouble

Hi there All

For a while I see a strange oil pressure indication on my EMS (MGL Xtreme2) : everything looks ok until half to one hour of flight. Then the indication goes from 4,5 bars (normal) to 9 or 10 (or even I saw 12) and erratic. It goes back to normal at landing (low RPM).

I changed the sender and got same.

On an another hand I measured compressions and all seems ok but the fact that I ear little oil burps when I apply 6 bars.

What do you think ? Low pistons ring leaking and pressure goes in oil circuit (under side of the pane is quite brown and oily)…