I live near the Windsor-Detroit crossing - the busiest in North America and we regularly use the numbers in our postal code (N0R 1G0) and add a couple of zeros as noted above.
Thus, as
@gmtech94 says, "N0R 1G0" becomes
01000 and it works fine.
At the Ambassador Bridge gas bar and duty-free store, everyone used to enter "00000" and also worked. Few locals buy gas there anymore as the regular stations in the greater Detroit area often have prices that are lower than at the "Duty-Free" store and giving
Manny-the-Bandit (the owner of the Bridge) any money is not appealing.
Within Canada, the best fuel prices are almost always at large gas bars operating on Indigenous reservations (Indigenous people do not pay many of the taxes forked over by the rest of us).
Near my area in SW Ontario, that would be the big modern Bear Creek gas bar on Iona Rd. about 6-7 km north of the 401 just west of London, ON. I filled my car and two jerry cans there last night and paid $1.40 / litre which saved me save about $25 over the retail prices at stations off the reserve.
Finding these Indigenous-run places can be a challenge but the locals all know where they are and the fuel is top-quality IM experience.