If it did not leak before and had no gasket sealant you may one of the lucky ones and have flat surfaces and not need sealant.
I like a lot of people have to try very hard to make the primary chain case oil tight, I make my own gasket out of 0.8mm thick paper, and use loctite flexible master gasket applied to the both mating surfaces, I also have a couple of case bolts with the heads removed to assist in sliding the case into position without messing up the sealant.
I use rubber washers under the bottom bolt heads as they leak on mine.
I use sae 30 briggs and stratton mower oil in the case mainly as its red and you can see whats leaking!
There are quite a few different ways to seal the case some manage with just silicon and no gasket.
Its important to remove all old sealant without scratching the mating faces, which is why using a gasket with no sealant and just grease is best from a time saving view, if you can get it to seal ok doing that.