I was taught a long time ago:
"A pair and a spare"
It kinda applies to everything and preparedness, and is self-explanatory.
We try and keep X amount of supply in stock at all times in case there is an unforeseen immediate need for that stock. And we order replenishment anytime we get below X amount of supply, not when we've used the last of the supply.
We apply that to oil, filters, belts, bread, soup, flour, sugar, toilet paper, you name it. It makes life so much less stressful, and does not cost any more in the long run, as we're going to use it anyway.