2. I wouldn't bother. Enough folks have run these engines with a gallon or two of RUG without issue.

3. Shouldn't be a problem. IIRC the 86-87 SDL runs a 2.88 diff. You'll give up cruising mpg in favor of livelier acceleration.

4. The tach connects to the 603 flywheel sensor. IIRC the signal goes thought the EDS module. I don't remember if EDS does something to the signal or simply passes it on. You're running the Diesel tach, right? The gasser tach won't properly display the flywheel sensor signal. IIRC you need a Diesel Klima relay and cruise amp to operate those features.

Sixto
87 300D