Oily gasket surfaces make the gasket stick. Always clean the thoroughly. I always also apply some copper or graphite grease on both sides of the gasket - never had a leaking problem, and it makes future removal a lot easier (does not stick). A trick on older airhead BMW valve covers was to apply oil to the cover side, and copper grease to the engine side - that way the gaskets stuck to the cover when removing it, and it made valve adjustment slightly faster.
If you want a really good seal, some hylomar on the gasket also works well (and is also easy to remove in future).
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