-40 will do it or even -50. 0W-40 and 15W-50 are commonly used synthetics, and 10W-40 or 20W-50 otherwise. This isn’t an exhaustive list. There aren’t a lot of reasons to use -30 or others but I doubt you’ll damage the engine.

At least 91 octane. 92 and 93 aren’t easily available in California and MBs don’t blow up.

About 85*C or closer to 80*C than 100*C. These gauges are more dynamic than typical American or Japanese car gauges that get to an operating point and never move. You might notice the gauge creep towards 100*C in heavy traffic on a hot day or pulling up a long grade at speed.

Probably normal given the electronic throttle actuator. You can’t really change idle speed.

Difficult to say but my guess is worn differential mounts. Peek at the differential from behind the car. There will be two mounts directly into the differential rear cover. The bolt heads should be centered in the rubber bushings. If the bolt heads aren’t centered, the bushings have collapsed. It’s an awful job because the differential has to come off or at least move clear of the bushings. The noise won’t get a lot worse so there isn’t any urgency. If the rear suspension needs work, you might as well drop the subframe. The bushings between the lower control arms and bearing holders also wear out on these cars causing creaking sounds when loading and unloading. It seems they’re ready for replacement the day after you install them.

Did you get a pre-purchase inspection? If not, look for a reputable independent and pay $100-150 for a health check.

Sixto
98 E320s sedan and wagon