Silolene has seen my X Country to 50,000 miles, trouble free. I use it in both the Country and the Challenge. Reasonably priced and I change every 5000 miles, because if the speedo has a 5 or 0 on the thousands, it needs an oil and filter change! Don't bother getting a K&N washable air filter, the paper ones are only a tenner and can be gently blown out so last 20,000 miles in this country no problem.
While you are doing your service, change the FUEL filter. If you are on facebook, join the G650 X Country page, on there I have done a step-by-step, with pictures, on how to change the fuel filter (Mahle 315 from memory - check my number!) I bet yours is dated 2006! Dealers do not change the fuel filter.
Fuel filter is under the seat, housed under the 'big circle' that is looking at you when you take the seat off.
Undo the electric plug carefully.
Undo the 2 screws that hold the fuel line on to the 'lid' - DO NOT try to get the fuel line off the fitting, they are brittle and prone to break, the 2 self tappers are a much easier and failsafe way of removing the fuel line.
Tap the big ring gently to get it started unscrewing. I use an extension bar as a drift. Once you have it started 1/3rd of a turn it will unscrew by hand.
The ring lifts off and the whole lid comes out, there is a rubber sealing ring too.
Take the whole lot out.
Undo the 2 earth wires on the bracket.
Undo the fuel pipe clamp - I replaced with an 8-13mm screw up clamp ring (Jubilee ring)
The fuel filter just wriggles out with gentle force.
Stick new filter in the hole... replace everything. Jobs a goodun. I have never heard from anyone doing it themselves finding a filter not aligned with their date of build....