Here's my $0.02.
You're doing all the right things to make your gas car run as far as it can possibly go. I have driven gas and diesel MB's. It wouldn/t surpricse me to see you go 300K. But I believe that the odds of seeing 300K on a gas engine are considerably lower than seeing the same on the comparable diesel engine. I have a 240D that went 516K before coming apart, I'm now changing engines as we speak. I have never gotten close to this with a gas engine even though I take just as much care.
Here's why. Basically gasoline is a solvent, diesel fuel is a lubricant. The combustion byproduct in a gasoline engine is carbon, a mild abrasive. The combustion byproduct in a diesel engine is an oily substance with some soot, not as abrasive.
Also there are certain ills which you see in gas engines that you rarely or never see in diesel engines, such as burned valves and the like which cause the engine to be taken apart. A diesel engine tends to keep going even when it gets so worn out that you can hardly keep oil in it. The difference here is that a diesel engine BURNS the consumed oil rather than having foul spark plugs and the like.
Basically diesel/gas is like orange/apple.
Please don't let any of this discourage you. Your are changing oil often and keeping an eye on your engine. It also sounds as if your miles are CRUISE miles. I drive 1,000 miles per week myself, and change oil every 3,000 miles in my gas and diesel cars. If you maintain your dilligence(and thus your car) I think 300K and beyond is attainable. A lot of it has to do with luck as well. Remember you have one of the best gasoline engines in the world.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in