One is 5w40 the other is 10w40. All this info is in the last few pages mate, maybe have a read over them?
I do usually agree with you mate, but what is it I'm not getting? I do the filter up hand tight, not as tight as i possibly can but still pretty tight, smear plenty of old oil on the seal. And when I go to undo it probably have a bit of oil on the hand/fingers from undoing and catching the sump plug, and shockingly the thing doesn't want to come off. The nut means that when it doesn't want to come off you whip out a spanner rather than drying everything off and trying again by hand.
You are right though, I am probably overtightening it and I am not an expert, but i sure as hell wouldn't want to put it on to loose. The filter with the nut just makes it easier, we all know that it can be hard to undo things on cars from time to time.