Guys ....
The problem will almost always come back if an AD does the job ... All they do is repair to BMW specs, which will be to replace the gasket, apply sealant
at the position specified on the gasket (this point is relevant!) then do it all up again. Same gasket as OE, same assembly techniques .. same problem!
I did mine recently - and I ended up with more oil on the motor than I started with .. HOWEVER .. I'd caused that problem by being a tw@t and I'll explain how...
The cam cover retaining bolts are shouldered which means that they can only squeeze the gasket by a given amount. BMW specs dictate that sealant compound is
only applied to certain parts of the gasket - AFAIR just around the 'half-moon' bits, so if the gasket isn't squeezed enough and there's not enough sealant on it, IT WILL LEAK AGAIN!
When I did mine a few weeks ago I used a Loctite Flange Sealant (574 - an anaerobic compound, Google it to check performance specs etc - it's impressive stuff!) on both sides of the gasket and along its entire length. not just round the half-moons. The mistake I made was assuming a simple butyl 'O' ring on top of the shouldered bolt sealing washers would give enough extra downforce on the cover - I don't know if it did or not, but when I investigated the 'new' leak, I found that one of the 'O' rings had slipped to the side of the bolt effectively leaving a furking great gap under the bolt-head.

oil everywhere ... I didn't disturb the cover again, but took out the cam cover bolts, removed the rubber washers and installed a plain stainless steel washer underneath the bolt-head before re-installing the rubber washer. I used a smear of the 574 sealant on those faces and surfaces too, just as a belt-and-braces job. The washers needed a 10mm inner hole so I had to drill the ones I had to suit, but you might be able to buy them as needed from your local fixings supplier. The washers were 1.5mm thick, which effectively decreased the depth of the shoulder on the bolt which gave a corresponding increased 1.5mm of squeeze on the gasket.
I've done a few hundred fairly hard miles since, and no signs of any further leakage.,
There was mention of the possibility of the cam cover being distorted that causes the problem - IMHO it's not - the gasket is about 5mm thick so is easily able to accommodate any warpage, but in all honesty it's a pretty substantial casting so I doubt it's going to spontaneously twist.
If you're doing the job yourself, just be generous with the sealant on all mating surfaces, use additional washers on the boltheads and you'll be fine...
Greg