I also had issues with the rubber rocker gasket so I'm using cork.

Tip for fitting the rocker cover gasket: Apply a thin layer of grease to each side of the cork gasket before fitting. Use no sealants.

Tip for fitting the sump gasket: thoroughly clean the sump and block, take the new gasket and super glue it in place on the block, holding it there with the bolts. Once the superglue is dry, remove the bolts and fit the sump, torquing it correctly.

Lee