Oil consumption, ML320, 98

[per300]

Greatings from Beirut Lebanon.
First post here, seem to be very informative site.

My wife have a ML320, 98 model with 89000 miles on the clock. I have noticed that when driving downhill, using the gear for braking, it is using a lot of oil.
Normally this should indicate bad valve guides, however, I read on this site, that it might be caused by a bad inlet gasket. (I can not find that thread again).
If this is the case, what's the replacement gasket #.

Regards, Per.