My personal preference is Castrol 20-50 (though a different viscosity may be preferred in Alaska winters) and a Mann oil filter.

A new or first time owner should take the vehicle for an ATF service to a Mercedes Benz dealer or better yet to a trusted Mercedes Benz only, independent garage. A little too complex for even the ambitious DIY guy to try as a brand new/first time Mercedes Benz W124 owner. (Possible DIY risks far outweigh the benefits here, )

When I was in Portland about 8 years ago, I was referred to Burback Motors, they are east of downtown across the river, below Lloyds Center, as I recall.

The owner (Mike Burback) even had a W124 AMG as his personal car, so his shop should be perfect. The prices are VERY reasonable too.

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)