I use synthetic grease and have no leaks. No silicone is needed. Make sure the cover is flat, use a new gasket and don't over tighten the screws. Petroleum based grease works but does not last as long as the oil will dissolve it over time.

Hans & Bruce
Thanks again. I've forged ahead & used silicone on the outer cover & grease only on the inner. On my bike the gasket always comes away with the outer piece on dismantling so I've made that the semi permanent joint. If it's still a leaker I should be able to work out which joint is failing. The silicone needs 24 hours to cure so I haven't filled the case yet.