The oil tank would be my choice. It has a nice stable consistent mass of hot oil and easy access. A pressure indicator in the oil gallery is more important since motoring is your gig, It's a reference point so you know the oil supply temperature heading for the engine is reasonable or has not changed dramatically, granted there are hotter measuring points. You can buy glue on sensors for the oil tank. Where a problem lay is if there is no oil flow to the tank then the oil would be cool. Maybe glue a temp sensor to the oil filter housing?