More than 1000 gallons of paint is/are sold each day [duplicate]
Your friends are right. The verb should be are.
This is because the sentence is about gallons of paint, and there are 1,000 of them. Emphasis on total volume or individual containers doesn't come into it.
Compare with
A 1,000-gallon quantity of paint is sold everyday.
Here, the sentence is about a quantity of paint, and there is only one quantity so the verb should be is. Whether that quantity is a 1,000 gallon tank or 1,000 one-gallon cans makes no difference.
The subject of this sentence is more used as a pronoun.
a greater or additional amount or degree:
tell me more
they proved more of a hindrance than a help
As a pronoun, more has a collective quality. In some cases, a singular is probably mandatory
More than this quantity of 1000 gallons, is needed. [quantity conveys a singular amount]
In other cases, the plural form is probably needed
More than these 1000 gallon cans are needed. [these emphasizes the plural quality]
The term gallons is not the subject of the sentence but a modifier (quantifier) of more. While gallons plural nature may incline one toward a plural verb, as has been pointed out in comments (see tchrist above), a numerical term in front of a noun does not necessarily eliminate a collective singular sense. See also this discussion of dozen.
I would go for "is" instead of "are".
The word "paint" is a singulare tantum and a mass noun, which is a word only existing in the singular form used to describe a mass of something uncountable. In English, words describing liquids (water, paint), gases (air), powders (flour) and some materials (wood) are such words usually only used in the singular form, unless you refer to two different types of something, e.g.: Which of these paints do you prefer?
In your sentence, "gallon" is a measure word and not the subject, which grammatical number defines the correct verb form (is/are). "More than 1000 gallons" is a quantifier, which specifies the amount of paint and not the number of paint(s).
So, paint is still singular and it is IMHO more correct to use the verb form "is".