I changed my oil & filter today. When I refilled with oil the dipstick was showing that it was overfilled even though I hadn't poured much in. I removed the drain plug to let some oil out and only a trickle of oil came out. I poured more oil in to see if it would run through the engine but it didn't. The dipstick is now showing about 3 or 4 inches overfull but there is nothing in the sump as the drain plug is still out. Obviously there seems to be a blockage of some sort. Anyone got any ideas where this could be and how I can clear it?