I have been using the recommended straight 30 weight oil in my JKSON for about 500 hours. The received wisdom is that the non detergent grades allow contaminants to settle out into the sump rather than being carried along suspended in the oil. That is so, but I have found it also leaves a residue of hard to remove black goop everywhere in the crankcase.
I would also like to install a filter in order to use a detergent oil that is also a multi-grade for easier cold weather cranking, something I do nearly every day at this time of year. I would also anticipate less crud build-up in the crankcase with detergent oil.
Butch is correct, they may be had for small money from CMD. A filter kit can also be had here
http://www.stationaryengineparts.com/Oil-Filter-Kit.htmlIn the photo of the item shown one can see the Lister/Petter part number.
In my case, (I'm in Canada) I find that it is overall less costly to purchase from SEP even they are in UK. Reason being, CMD ships by courier,the added brokerage fees and taxes end up to be an exorbitant total, and it seems the couriered items take forever to get here. SEP, on the other hand, ships via Royal Mail. Ordered items get here without customs fees, extra taxes, and in only a few days. YMMV, of course.
I am a fan of scrounging the 'net to find something that can be adapted, but this readily available option seems the way to go.
Cheers,
Hugh