-36°c oil pressure drop

Today I tried flying with a pipistrel virus sw 912iS with hydraulic prop. Pre heated engine and cabin. This is as cold as I’ve ever attempted to try and fly in. Had oil pressure drop after take off. Was fine during run up other then coolant temp alerts. I block the coolant radiator with some foam to slow air when it’s this cold and also half tape off oil rad inlet as well as the cyl cooling intake. When the coolant temp goes above the oil temp lane A will go off so had to reset this before take off. 

Oil pressure was at 55psi at take off. At about 800ft it dropped to 13 then slowly went up to 20 or so but when it dropped I turned to land right away and throttled back almost to idle. Landed without issue then shut engine down checked oil tank and levels. Fired it up and oil pressure was back to normal. Tried it about 4 times stop/start engine without problems but did not fly again will wait until warms up. 

Any thoughts or ideas what could have caused the oil pressure drop? Perhaps the oil at the prop got so cold that it jelled up and slowed the flow, or maybe got too cold at the radiator? It’s possible that the oil tank vent somehow got blocked as I had my cowl flap open where the vent is during take off as my temps were getting warm on the coolant. Oil temp at take off was 90c coolant about 90c as well. I’ve flown in -30c before without this happening but this is slightly colder.