question

I would start with a chain of queues in classic 3D modeling with connections. Each queue gets a possible maximum content of one. And all are connected to previous queue by a connection of exit of port number one. Each customer or in model speaking language an item has the free will to occupy a queue. A label at this item tells a queue whether to hold the item in the queue or not. You achieve this by command of holditem or by a return value of -1 in Send to Port function.. Last queue of such a chain of queues calls holditem on every entering item. This will keep a next entering item in it. The retrieving process of fuel is a delay of creating an event in the future, which will release an involved item to an exit port of number two. For example you can achieve this by a delayed message or secondary process of processing another item in a processor. In a case where you want a customer to exit the chain over last queue, you have to set a value to above mentioned label which lets pass the item in all queues of the chain instead of releasing it to output port number two. The exit connections are going to an object which represent your exit.

once your logic works as intended you replace your item by operator item class and add a logic procedure which uses this item as a transport to get to next object. You find such procedures in Use Transport option picklist items.

A different approach replaces queues by processors. But then you control passing pump station or receiving fuel by process time. If an item passes then its process time is zero. If it stays it has the actual processing time. Depending on exit strategy you send the item to next processor to output port one or two.