[TWinOKC]

Did the second oil change today.   When I replaced the filter, I think I left the small O-ring out.   Is the small O-ring necessary?  Looks to me like the two large O-rings on each end of the filter are not critical either, just used to align the filter.  The large one under the cap is the important one?  It keeps oil from leaking out?

(I have not started the bike yet)

Thanks 

[r80rt]

I'd put the O ring in.

[drbvac]

I sure wouldn't want to expect that it is there as an optional seal for something - I can stop pretty well with the rear drum brake but probably wouldnt leave off one of the front brake pads

[Arizoni]

TWinOKC

I don't think the small O-ring does much but the two larger ones on each end of the filter cartridge seal off oil leakage at each end.
This is important because without those two larger O-rings the oil can bypass the filter letting unfiltered oil pass on to the engines bearings.

The outer O-ring seals the cap with the case preventing oil leaks from dribbling down the side of the engine.

My G5 also has a flat gasket between the cover and the sidecase and the only reason I can come up with for that gasket is it is one way to adjust the spring pressure on the relief valve spring.  As I mentioned, the O-ring in the groove actually seals the filter cavity.