Mercedes sprinter 2012 mdb906?

Hi. I have a Mercedes mdb906..sprinter.
Has an oil leak it seems from between the oil filter housing and the engine block..
Very slight, however, it gets on the serpentine belt , and then every where else..
Is this common fir the v6..
Will a New gasket do the job..
Are there any special things to watch for when changing filter, I always change the o rings, toque the cover..

Also what is the best way to flush the auto transmission..
In the morning it hesitates to engage as I pull quietly away.. done 475k kilometre..
Unsure of previous servicing..