ballooned oil filter
drain the oil. leave the oil filter on. put 1 gallon of diesel fuel in it. let it idle for 30 seconds. drain the diesel back out and put new oil (5w-30) and filter and see what it does. relief valve could just be sticky from not running in a very long time. If the filter you took off wasn't ballooned, that tells you it wasn't sticking when it was parked