Another problem is, I have noticed on these 98 w210 is the plastic
I replaced my oil filler neck and that solved the oil leaking problem, just replacing the gaskets didn't work. The valve cover is bone dry again.
I change my oil every 6 months (3K miles) with no loss of oil on my 98 E320. I use to use Mobil 1 15W50 in the summer and Mobil 1 0W40 in the winter. Now I plan on using the 0W40 year round. On the 98 E320 the manual states that the engine oil capacity is 8.5 Qts. I found that using 8.5 qts tends to over fill the crankcase with oil expecially when the engine is hot. I've been using 8 Qts of oil which puts the level just under the max level with the engine hot. Keep in mind that there is a 2.5 Qt difference from the min to max level.