As far as I know Delco, Wix, and the other big car players don't make filters that fit the GSXR (except for Fram which you don't want to use). This is most likely due to the oddball thread size (20mmx1) that doesn't cross-match to any cars or even many bikes.
I don't have part numbers for most of these but I know of the following filters out there:
STP: Like Cherry said you can get these at Autozone and look very well made when you pull them apart (I have). I've used them and would again.
Fram: Forget about it. Commonly available, not particulary cheap except if you mean the way they're made. Cut one open and you'll see why no one who cares about their bike should use a Fram filter on it. Crap crap crap.
Perf-Form J509: Small company that specializes in motorcycle filters. Not many places carry them direct but you can order them directly from www.perf-form.com. I've never pulled one apart but I've used them and I trust them. They're also made in the USA if that's important to you.
Emgo and Vesrah: Mass market aftermarket motorcycle suppliers. I've never pulled either one apart but I just put an Emgo on my bike. Available at some bike shops and a lot of mailorder places.
Those are the big ones there may be others but I haven't seen them.
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by jeff: i'm fairly certain the 86-87 oil filters are different than the ones used in later bikes.....if i remember correctly, the 86-87 filters are substantially larger.<HR></BLOCKQUOTE>
I did look at my book wrong. It is the MO-18 for the '88.