Tipping a filling station attendant is not generally expected in the United States or Canada. Some guides flatly deny it is ever expected, though others acknowledge there is disagreement on the matter.
Anecdotally, in all the times I've been in Oregon and New Jersey, I have never seen or heard of anyone tipping the attendant simply for pumping gas, except to leave them the change when paying cash (I have not used a full-serve pump anywhere else in the US or Canada in over 20 years, and cannot even think of where to find one).
Fifty years ago, a good attendant might have checked your oil and coolant levels, wiped the windshield, and checked your tire pressure in addition to filling the tank. An increase in electronic sensors and decrease in drivers' patience over the years means such attention is rare today. In most places, "full serve" is not really full service, and simply the term used to indicate that an attendant is working the pump.
With that noted, except where banned by local ordinance, attended stations charge a higher price, which is understood to cover the wages of the attendant for the basic work of filling the tank and collecting payment. Moreover, unlike a food server or barber, or even barista, there is very little social interaction with the attendant, and tipping usually accompanies more intimate or longer-term interactions. As an etiquette columnist wrote for a Boston Globe "Miss Conduct" column:
Tipping is customary only if there are both full-serve and self-serve options at the station and the fuel is priced the same at both.
This would exclude areas where self-serve is banned altogether (including the entire states of New Jersey and Oregon), which is borne out by anecdote— the only people who tip in New Jersey are out-of-towners.
But when extra service is rendered, some tipping may be in order— various guides suggest $1–2 for cleaning the windshields, and at least $5 for any work involving getting on the ground or checking under the hood. As etiquette authorities will note, tipping someone working in a service job is always appreciated, and never impolite. Pumping gas is a monotonous, fume-infused job which is at best unpleasant, so I would give some consideration particularly when the weather is hot, cold, or wet.