The owner's manual for my '91 300E says to use at least 91 octane fuel and if you can't get it you can use SOME lower octane UNTIL you can get the right fuel.
If you use lower octane, you are told to not exceed 3,000 RPM and to take it easy until the correct fuel is obtained.
The cost of fuel is insignificant in the total cost of car ownership.
BTW, I live in the Phoenix area where gas is nearly as costly as it is in California and (recently) where getting premium was almost impossible. Nevertheless, I never considered using the wrong fuel.