[Sven7]

If you look at BSFC charts, your engine's most efficient operating speed is likely not 1200rpm. Usually it's more around 1500-2000rpm at about 80% load. I can't say whether going 30mph in fifth gear is good for your engine (even if it's not lugging), but if you're going that speed you should probably be either accelerating through your best BSFC, or coasting in neutral. That is, if you're looking for the most fuel efficient way of driving.

As for being in 5th or 4th, the higher gear is almost always better if you can pull it off. The question is whether it's prudent, considering you make more hp for each drop of gas when you're in the BSFC sweet spot. You'd have to find a chart and compare it to your driving habits to find out for sure.

My Rabbit doesn't like to go under 40mph in 5th, but it doesn't necessarily shudder. The Probe will happily go down to about 38mph in 5th. I don't feel that driving slower than that is safe on the highway, anyway. In town you should be coasting at those speeds when possible.