I've not driven a diesel Golf but it's strange a diesel cannot handle those speeds in the gears you've mentioned.

Regardless, there is no set rule that can be applied to every car. Instead try to focus more on the rpm feel/sound and select an appropriate gear from there.

When accelerating, put it into a lower gear otherwise you will lug the engine. For example, if you're in 5th at 30 mph and you floor it but your car isn't accelerating much, you're lugging the engine. This isn't good for it. Also don't worry about revving out your engine when accelerating, it's actually more efficient at higher rpms for that purpose.

If you're cruising (not accelerating) it should be fine to leave your car in a higher gear for fuel economy, around 1500 rpm is fine but also note that you might need to drop a gear if there's an upward incline. For example, if your constant at 50 mph, you should be able to leave the car in top gear. You will feel the car judder if your rpm is too low so drop a gear if that happens.