Is it possible to use a F oil cooler for the older 91? If so where do the oil filter fittings bolt on to?

In short, No.
Not in the sense you are thinking I'm sure...

One member has made a kit to put an oil cooler on older models, but I'd imagine its more pricey than you were considering...

http://gstwins.com/gsboard/index.php?topic=62499.msg730395#msg730395

- Bboy

I was afraid it would be like that. Thanks for clearing it up.

Why do you want the oil cooler?

Pretty much to get an oil cooler, you change the engine to a 2004+ engine.
Oil cooler added for the "F" model with the fairing.
Elsewhere in the world, the 2004+ models were available without the fairing. Those naked models called GS500E (yes - "E") did not have the oil cooler.

Thanks. I went to the website in german and he described that very well wit the aditional oil channel in the block casting.