I know this is an old thread (and that I haven't posted in far too long), but I just wanted to update on this.

When I resealed, I did both sides using the Mercedes sealant I mentioned above. The driver side is still going strong, but the passenger side started to leak again after a couple of months (which makes me believe it was a case of me not doing it properly).

As I have been very tight on time over the last year due to work, I took it to a local mecanic I trust who used the gasket sealant he recommended (I did make sure to stress all the points about it being "sensor safe" etc.). That was about a year ago now and no problems since, although I have only done about 1k since then.