If you have a good tyre for mud conditions, the tyre will self clean so build up is not an issue. I use an E09 rear because I can get 5000 miles from it as opposed to 2500 from a C02. E09 front isna much good on slippy stuff and the best I've tried for these conditions is the Metzeler Unicross, it's brilliant and not bad on tar too. Oponeo have these for 40 quid.

__________________
Touratech big guard. Yamaha handlebar guards. Pivot Pegz. Yam main stand. Camel Toe. 30 mm risers. 14 tooth front sprocket. Big balls. Old and not wise.