Pipeline

Yes — you may place your worker on any Action space in the center (or on Government Pipe tiles or in your own Pipe Network) regardless of whether another worker is there, provided you can pay for the Action (p6).

If you are both purchasing and selling in a Market you must complete all of your selling before purchasing; you may sell a higher grade as a lower grade and may trade as many barrels as you wish (p7).

When you take a Contract you immediately choose Active or Deferred; Deferred Contracts become Active at the start of the next year during the Refresh phase, and you may not defer Contracts in year 3 (p7; Refresh step on p6).

No — each Pipeline may only refine one barrel and you cannot run the same barrel through multiple Pipelines in the same Action; all Pipelines run simultaneously and each may only be run once per turn (p9; p10).

Machines do not count as a section of Pipe and when placed on a pipe tile they bisect that Pipeline; during the Machine Phase machines refine oil through the Pipelines connected to them and all Machines run once simultaneously (p8; p10).

You only need a free Tank for the final grade — you do not need tank space for intermediate grades — but you must have a free Tank on the corresponding row to move the refined barrel to, otherwise it cannot be refined (p9).