For a specific Mobil 1 weight...if I were putting my car on synthetics I would likely switch to 15-50 Mobil 1 fully synthetic. Most parts stores sell it for about four bucks a quart and twenty three bucks a case. I probably would put the 500E on synthetics but for its six figure age. It actually has a cleaner valvetrain than most cars with 30K miles but... It has done so well on 3K mile 15w-40 QS changes so I may stick with what works. For your 99 E320, give it 10 or 15 thousand miles to "break in" on conventionals, then go synthetic.
PS Your dealership should be able to show you the same chart they had printed in the Star. I've seen that same chart before in various MB literature. Odds are he should have some of that on hand.