If it makes any sense in looking at it, usually this stuff gets smeared on the bottom of it, then just put it in place and screw 'er down, no need to wait. If it's what I think it is, it is to help the unit transfer heat to the chassis, helping the unit to keep from getting too hot.
If MB was as smart as they think, they'd have mounted it on the side of the windshield washer tank.........
Gilly