My car is a W123 diesel but ....

I too have also had problems sealing my vc gasket when using aftermarket gaskets including Victor Reinz. Last valve adjustment, I splurged for an MB OE gasket. Leak easily gone and I'm not looking back from now on. Cost is about twice that of a Reinz but still at around $20 it is a bargain to keep your engine clean.

I used to use a coating of assembly grease to hold the gasket in place while refitting the cover. The MB OE gasket fits tightly on the cover thus not needing any sort of assembly aid. I have never used a gasket sealing compound and don't believe in them. The surfaces, particularly on a Benz should be machined well enough to not need any help.

I guess you know to not overtorque the hold-down fasteners?