We are planning a backpacking trip to Norway and plan to use our MSR WhisperLite multi-fuel stove which runs on white gas or regular unleaded petrol. Will gas stations in Oslo sell us such petrol if we bring our own fuel bottle? I saw there is a petrol station not far from our hostel.
You could look for Aspen fuel. This is essentially a brand of petrol without additives, so it should burn a lot cleaner. So less problems with the stove clogging up, and much less toxic emissions. It is similar to white gas, aka Coleman fuel.
Note you need Aspen 4 for a stove, not Aspen 2, which is pre-mixed with 2-stroke oil.
Aspen is a Swedish company, so hopefully it should be widely available in Norway. The Aspen Norway website has a dealer finder, it lists a number of options around Oslo.
The MSR WhisperLite should also be able to run on kerosene. If it is not very different from other kerosene stoves, you can run it on lampeolje (lamp oil), which is easily available from larger super markets or petrol stations in 1l bottles. It is very similar to kerosene, but will burn with less soot. You may also find a similar product called fritidsparafin (literary: recreational kerosene).
Many petrol stations also sell regular kerosene (parafin in Norwegian) and you should be able to buy smaller amounts of both kerosene, petrol or diesel from the pump at any petrol station. The pumps are often marked with a minimum amount (2l or 4l), but that only means that the calibration of the pump is only guaranteed to be within legal limits above the mentioned amount, not that you are not allowed to buy a smaller amount. I suppose it can be very difficult though to fill a small fuel bottle from the pump nozzle without making a mess.