Take the oil filler cap off and have a look at the rubber o ring seal around the top. There shouldn't be any cracks etc. If it's there and it looks ok, give it a wipe, carefully wipe around the clutch cover where the cap screws in and then put the cap back in. It should be snug but not stupidly tight. Mine usually dribbles about after I've changed the oil (easy Tom) where a bit of oil sits on the rim of the hole.