I like Mobile 1 if you want to go with synthetic. Synthetic oil is better oil molecules that take longer to break down.
For regular oil I use 10W-40 in the winter and 10W-50 oil in the summer. If use anything lighter, it seems to burn up a little too fast than normal. My manual says that I should burn 1 quart of oil for every 2,000 miles. I monitor my performance.
If you can afford it, go with the synthetic. If not, go with the regular and change it more frequently.
Just my 2 cents (pounds).
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)