Just yesterday I found the replacement gasket that had been fitted into the cap on mine to cure an oil weep had split and was ready to drop. I've got an early car with the dipstick in the oil tank cap so I've cut a new gasket from nitrile rubber and extended the thread on the top of the dipstick far enough to get a lock nut and large washer under the gasket to secure it in place. The dropping gasket has killed at least one engine as far as I am aware.

Calum
M3W Vol1
Vol2