KD, the extra o-rings are for the older 500s and maybe the classic 500(?) Yep, the 5-pack is the most reasonably priced. Now, if this is an old school bike properly broken in and running clean, and isn't just dogged to the point that dirty oil is the usual thing, there is no reason why giving it an oil change without changing the filter can be safely accomplished. I've been keeping close eyeballs to the oil, and with the last filter changes I'm impressed-I backflushed the filters and gotten very minimal 'crapola' out of the paper. Granted the bike only has 3100 miles on it, but I've seen worse out of scooters more gingerly used. The oil/filter change was done on this bike every 300 miles until the last one at 2400 miles. Now the target mileage is going to be every 2100 miles for oil/filter with an oil change at 1000-1100 mile intervals. Of course, driving conditions vary, and the intervals will as well.