My XTs have never consumed oil between services. Indeed NONE of my Yamahas have ever consumed oil. I use 10/40 and I switched from Castrol to Bardahl a few years ago and both are excellent. And ps I have always used a little Redex or STP in the petrol every few months, as this is one way of cleaning the injection system and the top end, I would suggest this could help your problem maybe?????

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(Now sold on, sob) 2011 Blue XT660X with gold wheels, was 26,500 km.
Engine mods: K&N Stage 1 filter, DNA Stage 2 filter, snorkel removed, Kev fuel mod fitted.
Plus: smoked Puig screen, Yam aluminium sump guard, Yamaha / Acerbis handguards, Givi Trekker aluminium side cases, Leo Vince X3 cans plus Kev front fork mod and Fender Xtender. Just fitted Michelin Road Pilot 3 tyres.

SOLD the XTX for a Super Ten 1200. And just bought a Raptor 700 so staying on here for some tips....