Paint, like water, is a liquid and, therefore, like water, noncount: "paint (noun) 1 : a liquid that dries to form a thin colored layer when it is spread on a surface [noncount]." link.
In biomedical texts, when we use units of measure, e.g., milliliters (ml), the sentence always uses the singular because the quantity of liquid is considered a singular unit. When the sentence says aliquots, which is a counter, then it uses the plural because aliquots is plural: ...two aliquots of blood (approximately 20 microliters each) were transferred... [ABSTRACT, line 5] and When a number is used to describe a single measurement, a singular verb is used (10 ml was added). However, if the subject is considered in separate parts, a plural verb is required (10 ml drops were added one by one). [Grammar #5] and What is the change in vapor pressure when 164 g of glycerin (C3H8O3) is added to 338 mL of H2O at 39.8 °C?. It's the same with dollars: "One million dollars is a lot of money". While dollars is a unit of measure, it's not a counter, as bills would be: *One million dollar bills are more than I can fit into my backpack", but *One million dollars in bills is more than I can fit into my backpack". Sure, English speakers say things like Give me a water instead of Give me a {glass/bottle} of water, but that's merely an elided sentence, not an indication that the speaker thinks that a water is a discrete unit outside the container.
Gallons are a unit of measure here, not a counter. Therefore, there is no problem with either 5 or 6: (5) More than 1000 gallons of paint is sold. [emphasis on total volume] (6) More than 1000 gallons of paint are sold. [emphasis on individual containers].
Wiki Answers has a slew of pages that ask this question with various quantities of paint: How many gallons of paint is required to paint a 5000 square foot house?. Obviously, not everyone agrees with those who believe that gallons of paint is always plural. That belief, like many others about English usage, is strictly an article of faith. Educated native speakers disagree on these seemingly basic points, and those who insist that they are right and everyone else is wrong are simply being pigheaded, dogmatic, and solipsistic.
Can you imagine standing in front of a crowd of educated English speakers and saying Fifty gallons of paint are a lot of paint to carry? I can't. If I could, then, like my wife, a native speaker of Taiwanese and Mandarin Chinese, and most of her friends and relatives, I'd also say things like How many money do you have right now? In Japanese and Chinese (and probably many other languages) there is no distinction between count and noncount nouns for much/many and, with the exception of pronouns, no indication of singular/plural for nouns and verbs. Not true with English, although native English speakers are trying their best to eliminate the less/fewer distinction: {There's/There are} less people in the store today instead of the old-fashioned There are fewer people in the store today. But this is merely a generational change in the language. It proves nothing but that language changes and that what sounds infelicitous to us old fogeys sounds normal to those who will soon enough become old fogeys complaining about how future generations use the language after we are long gone.
