So here's a new one, that I think confirms that my (slightly) rough idle note is fuel related, perhaps with the lift pump.

Started the car on a moderately steep hill, put it in drive as soon as oil pressure came up, and it died! The parking brake and main brakes were still applied, so it wasn't from load, it was entirely due to height differential between the tank and the motor.

Started it back up, waited a few minutes, and she drove off fine. I haven't had any issues at all with lack of power (cruises up as fast as I'd like on the freeway) and I just replaced both filters and the in-line filter fuel lines. Going to throw in a can of Startron next for good measure, probably drain the tank and clean the strainer once it's run down toward empty.

It's amazing what minor problems crop up in 30 year old cars... problems you don't see in most other vehicles because they don't last long enough to manifest.

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1992 300D 2.5T 249k - Parked with a bad transmission
1981 300SD 142k - Daily driver