If you use your car as a daily driver and you CANT get premium or Ultimate, you CAN run your car on regular unleaded as a stop gap measure until premium fuel is available again.

If you do run your WRX or STi on regular unleaded, the following steps are very important:

1. DO NOT load the engine up in a high gear, especially up hills IE full throttle 4th gear at 2000rpm will cause detonation.
2. DO NOT use full throttle at any time.
3. If you can afford it, use an octane booster, but still follow steps 1 and 2. You can never be sure how high the octane will be when using an octane booster, because all brands are different, although it will be safer than using plain regular unleaded.

Those with aftermarket ECUs have to be even more careful of these measures, because the engine will be in a higher state of tune.

Your engines are equipped with a knock sensor which will retard timing in case of detonation, but it wont prevent ALL detonation. Some aftermarket ECUs do not utilise the knock sensor, and more care will have to be taken.

Remember, this isnt the end of the world, and you can still drive your WRX until premium fuel is available again.