OM, I have a car that is "kinda similar" to yours - '91 300SE.
It has spent it's entire life in Texas; approx 180k miles, and I have owned it since it was a baby.
(Up-front disclaimer: Just my opinion, your mileage may vary, etc, etc).

1. Oil
I tried M1 for awhile, but soon concluded that the major benefits of synthetic oils are in super-tough conditions like very cold startups, desert towing, racing, turbos, or very long oil-change intervals. I don't do any of these in the old Benz (but I definitely run M1 in my modified Vette).
For the last 8 years or so I have been running Valvolene 10w-40 Max Life.
It is supposedly formulated with the viscosity at the top of the 10-40 range, and has additional detergents and seal conditioners. May be marketing BS, but it sounds good.
I have had no oil-related issues whatsoever in 180k miles - when we redid the head gasket at 110k, the cylinder bores looked like new, and there was no sludge at all. Initial startup is still where 90% of wear occurs, so I think the 10-40 is the best compromise for even a very "experienced" Texas engine. I fully expect my motor bottom end to go 300k. No matter what you use, the front timing cover will leak every other year.

2. Filters. I use a Purolator oil filter. They have the anti-drainback feature, and Pepboys always has them in stock. I suppose there could be better filtering ones, but I have no definitive proof of that; and no problems.
Same for the air filter; I buy a name brand paper filter. Everyone argues the benefits of the K/N, but everything I have seen on the Corvette Forum indicates the K/N replacement element (not the whole cold air kit) is less restrictive, will make a tad more power (1-2%) on a high-horsepower engine, but does not filter quite as well as a good paper unit. I wouldn't notice 1.5 HP on my M103, so paper it is.

3. Spark plugs. We all agree: NON-RESISTOR PLUGS ONLY.
The OEM Bosch H9DC is still the recommended one, but pretty hard (impossible) to find. I bought up a 10-year stock awhile back. I hear there is an NGK equivalent NON-RESISTOR one available - do a search here, tons of info.

Things you didn't ask:
- plug wires are stranded copper, and last a very long time if treated well.
- Ditto for the rotor and cap.
- Use the yellow antifreeze for Daimler-Chrysler cars. Autozone carries it - Zerex I think, but can't remember.
- change your power-steering filter, and use the MB fluid.
- When changing the tranny fluid, find the torque-converter drain plug also. You can get 7/8th of the fluid drained.
- After 17 years, belt tensioners die at the worst possible time.
- PBR brake pads DO NOT DUST, and have 99% of the same pedal feel.

Lots more, just check the archives.

DG