I've been wondering, what makes muffins less sweet when using oil? (instead of butter?) In this case, a muffin mix tells me to use butter. The alternative (to make "american style" muffins) is to use (sunflower)oil. It would make it less sweet.
Oil and butter don't have the same capacity to hold the sugar/sweetener used in the mix. Whatever isn't dissolved fully in the shortening will still be bound up in the final product (or maybe end up mostly in the outer crust), and you'll likely perceive the difference as more/less sweet.
The main cause is that butter has quite a lot of water bound up in it, which you wouldn't find at all in oil. You might get a closer similarity by comparing melted, evaporated butter to oil (1-to-1 proportion), since solid & melted butter have different volumes depending on how much water you've evaporated (1 cup solid /= 1 cup melted).
I've got an answer in another question that applies more generally to different phases of shortenings and perceived/actual sweetness - Does butter reduce perceived sweetness?
I think you are looking at it the wrong way... butter makes it sweeter, so not using butter is naturally "less sweet". It isn't that the oil 'causes' less sweetness.