So far I'm averaging 3 welder's oil per collection. Given that we can still collect welder's oil even though we don't need any for chapter 7, we have a perfect opportunity to get ahead with our collections for chapters 8-10. We need a total of 60 welder's oil for those three chapters.
It's a bit annoying to have to babysit the game a bit in order to collect every two hours, but it should be absolutely doable. And if anyone wants to throw money at the problem, there's always the possibility of buying a second fountain for 150 gems.