Sometimes, Putt Putt has to get food for a baby animal owned by Outback Al. A good number of them need milk (as explained by the book in the library). They're all different mammals, yet Putt Putt always brings the same kind of milk (cow milk), and it works. Although this decision was clearly made to make the game easier, it's still inaccurate - each baby mammal would need the milk from its own species.
It's a common myth that high-octane fuels can increase the top speed of a vehicle, and the game perpetuates this myth. They don't. Sports cars tend to use high octane fuels due to their high compression ratios; higher octane fuels are less likely to detonate unexpectedly at such pressures (which wouldn't be healthy for the engine). Putt-Putt would've noticed a very slight improvement in performance with higher octane fuel if anything.