As well as the oil seal conversion, I drill a small hole (6 mm approx) in the (rear) alloy chaincase up high, just below the magneto points cover, and fit a Yamaha DT series breather.
Oil doesn't seem to come out of a breather when located in this area, but the otherwise inevitable pressurization of the chain case cavity is prevented. The filler / inspection cap is also less likely to drip, because of the breather.
With the original scroll type of sleeve pressurisation could not occur, because of the clearance. Unfortunately, it could be leak free either, which is why we install an oil seal conversion.
Richard