OK, HaHa!! This is the way I understand it. The Yamaha 2stroke uses a filter for the gearbox only. It is the same shape and size as the 4stroke filters used to oil the gearbox and the head.
Have a look at these pics. Both are Yamaha oil filters. The filter on the left is for a 4stroke Yamaha. The filter on the right is for a 2stroke Yamaha gearbox(gearbox only). If you put the filter on the right in certain Yamaha 4strokes it will absolutely, completely waste the top end!
Look real close to the filter in this pic. See those 4 small holes along with the 2 large holes? The 4 small holes are what oil the top end!
Now I've never owned a YZF and I have no idea what the newer filters look like so do your own research.
I'm still wondering what yamaha 2 stroke uses an oil filter like that. Is it a vintage one? I've had 9 yamaha 2 strokes since 1998 and none of them have had an oil filter. I'm not sure about the vintage ones though. The YZF's use a filter that looks like the one on the right, with a big hole in the middle. The oil filter cover has a prong that sticks in the hole to keep you from putting it in backwards.