Hello , I've just been using neatsfoot oil but only on the outer surface and not on the pulley . I've used adhesive and countersunk head screws on the pulley lagging and therefore not wetted it but it holds ok . I have been in correspondence with a gent on your side of the pond , he makes leather belting for stationary engines and tells me the driving surface should be the outer , or skin side , not the fleshy side !! I have always riden with it the fleshy side driving assuming it is rough and therefor should grip better ! I did try the outer side but have put it back and assume rightly or wrongly that maybe stationary engines might run ok like that due to usually a more steady 'drive' and usually their belts are run much more loose tending to wrap themselves around the pulley more . Whereas motorcycles usually with smaller 'driven' pulleys probably rev a bit higher and accelerate more quickly than the normal stationary engine !??? Maybe ! I have just ordered a bottle of castor oil to try that . I also use a modern spray can of belt anti slip dressing on the inner side of the belt BUT it still slips at times .