Q: What kind of oil does she require? I use Castrol 10w30 in the 940. Would this be okay? How about a synthetic? Do the 'Mann' filters fit the B20?
A: Yes, that would be fine. If the engine is a bit tired (oil pressure not quite up to par) you might try slightly heavier oil. Mann filters fit an incredibly long span of Volvo's back to but not including B16 engines.

Q: How involved would it be to install an electronic ignition? Are there any significant differences between a '75 240 ignition system and the aftermarket ignition systems (e.g. Crane) except for price?
A: The Crane and Perlux just fit into the spot taken up currently by the points. I put a Crane XR700 on my car and have been really pleased with the results. I don't know much about the '75 240 ignition.

Q: I've read that 123GT's sport a tach as part of that car's standard instrumentation. I don't see a "stock looking" location on my dash for a tach, and so I'm assuming that the 123GT has a different dash layout. Is it possible to swap dashes or, barring that, a way to install a tach so that it looks stock?
A: On the 123GT the tach was a free standing gauge that just bolted to the top of the dash pad to the right of the instrument hood. So other than finding an OE Volvo tach you'd just have to drill a small hole in your dash to bolt one in. IMO (and in my 544) to the left of the instrument hood is a much better location.

Q: Sort of related to the question above...can anyone tell me what the top speed is in each gear? I've been shifting into 2nd at about 10mph, 3rd at 20, 4th at 35, and overdrive at 45. I'm probably shifting a bit early, but since I don't have a tach... Also, what is the B20's usable powerband?
A: The useable powerband depends on the cam in your engine as well as the breathing provided by the carb and the exhaust. Probably you aren't going to get much power above 6000 rpm or so with a single downdraft Weber (I'm guessing you have a Weber DGV kit on there) and a stock exhaust. The engines are tough, tough tough and you won't break one by revving it up - just use the area where power seems to taper off as a guide.

Q: What is normal fuel consumption for a single carb Amazon? I had the opportunity to fill 'er up yet, but probably will tomorrow morning.
A: There's a world of variables in that question. Just as a rough, rough guide I'd say you'd get anywhere from 20 mpg (spirited city driving) to 30 mpg (careful easy driving with lots of moderate paced highway driving). They aren't uite as economical as a newer car with similar power and weight because an old carbed car has to err on the side of richness in the mixture to ensure good drivability.