Do this on the autostrada whenever you can. It’s open 24/7 and attended.
Failing that, try to fill up on the superstrada, i.e. a toll-free highway with two lanes in each direction. But be warned! It was in a superstrada filling station that we met an angry Dutchman (almost a contradiction in terms). He had been charged extra by the attendant for “helping” him to fill up, without his permission. She had just grabbed the dispenser from him. I tried to find out what one was supposed to do by talking to the attendant, and it turned out that three of the pumps were permanently unattended. The staff had no access to them and you had to pay by machine. The other pumps were officially self-service, but you could ask the attendant to do it for you and that would cost extra (I think). Or were some of them designated “self-service” and others “assisted service”? Anyway, she was so busy talking that she forgot to charge for filling up for us.
Only use filling stations on other roads if they are attended. Do not, repeat do not, attempt to use the pumps and pay by machine when there is nobody there. First of all your credit/debit card probably won’t be accepted. Secondly, you need to have new, clean notes. Thirdly, it’s quite a complicated process. Fourthly, and this is important, the pumps sometimes run out of petrol/gas and you have to keep your receipt and come back next day. How you are supposed to do this, when you have run out of petrol/gas and are desperate by the roadside in the dark, I do not know. Also, you are not likely to come back next day if you are driving a long distance.
Oh, and it’s illegal to have a jerrycan of petrol/gas in the boot/trunk.