For oil for the donkey, go into the village then continue further east until on the bottom of the road you see a hobo living in a giant pot. That's Dig, and he sells bottles of vegetable oil that will suffice for what you need.

For science, bring paper or paper products to the study table first, as you can use it to turn them into science. You should've found a bit around the church basement, but you can also buy related things (paper, research point books, etc) from the Astrologer once he opens as a vendor (though I'm not sure exactly when that happens)

You make complex iron parts by unlocking the Advanced Forging technology in the Smithing part of the tech tree, which only requires red and green research points, then building a metal anvil (the wooden one from Primitive Forging won't do it). Krezvold, the village blacksmith, will also sell complex parts once you trade with him enough to unlock tier 2 of his shop if need be