What does the package say?
You are not paying for "1 gallon" of oil if the jug says "3.5 quarts."
This is nothing new. Grocery stores have been doing this with ice cream and other products for at least several years.
Very smart marketing people figured out that people won't pay higher prices for products, but if you put less in the package and charge the same amount, people don't tend to notice because they're buying out of habit and not paying a lick of attention.
Costs go up: Raw materials. Labor. Energy. Machinery. Transportation. Every year they go up, but people don't understand and expect stores to keep charging the same price for the same amount of product year after year. Sooner or later something has to give, or they'd just go out of business!
It is NOT "deceptive" as some think. The amount you are buying is clearly printed on the package. You're just not paying attention.