Problems like this are one reason I don't think it's practical to abstract things to "supply". To continue the "Oregon Trail" analogy, the game doesn't attempt to bundle everything into a single resource, but has a small but reasonable number of categories of items representing the things necessary to overcome obstacles. You can easily find yourself in a situation where you are supplied with one item but not with another more immediately necessary one. That's a huge part of the gameplay. Can you get away with selling your spare wagon parts in order to get more food? It's easy to imagine mishaps or events that concretely threaten one portion of the supply. If you have a generic thing like, "Find a fresh water oasis: replenish 12 supply", it doesn't really capture the problem of, "We still have water, but wild animals or insects ate our food supplies". It also doesn't really express how important that is if you have plenty of everything else but not the 1000 pounds of water the expedition needs. It also in an RPG context doesn't deal with returning to revisit a known oasis, since unless we apply some judgement one supply source like that could represent effectively infinite supplies. It's more realistic if we separate animal fodder, water, food, light sources and what not because then having a nigh infinite water supply doesn't mean you are just set for all manner of supply. Any attempt to abstract away these problems is just going to result in artificialities and complexities that mitigate against any value in "keeping it simple".
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