Subject and verb agreement is not that difficult to understand if, indeed, one wants to constructively appreciate it. And since language has never been substantially enough to comprehensively depict my thoughts about it, allow me to dissect the sentence “He discovered that those many gallons of petrol was not enough to take him to Sydney” so that, somehow, some issues could be eventually made transparent to those bewildered and parched learning minds.
The sentence pattern is S-V-DO, and the part we disagreed on is the DO “that those many gallons of petrol was not enough to take him to Sydney.” I would like to emphasize, as Maxsims would agree, the complete subject of the sentence is “those many gallons of petrol” but the simple subject really is “gallons.” Be it a single-word subject, subject phrase, or subject clause, none really is a problem so long as we know how to exactly spot the true and simple subject. The other parts are just modifiers making the simple subject specific in meaning and plain to readers’ understanding.
“THAT THOSE MANY GALLONS OF PETROL WAS NOT ENOUGH TO TAKE HIM TO SYDNEY…”
Before the simple subject, there are two modifiers, “those and many.” After the simple subject, there is this phrasal modifier “of petrol.” So, the main consideration for choosing the right number of verb should be the simple subject “gallons” and the two modifiers before it, “those and many.” “Many” originally is a pronoun and is always indicating plurality; “those,” is, at all times, indicating and emphasizing plurality and individuality to mean, as far as the subordinate clause above is concerned, that every one (1) gallon is considered valuable and significant to getting him to Sydney. In effect, the main consideration is not the entirety of the many gallons of petrol which can be considered a single unit, but, take note, each petrol-holding gallon composing and contributing to the many holding-gallons of petrol. “Petrol,” though it’s the operative subject, has not that much value in determining the number of verb that should be used and not the controlling word. Accordingly,the verb should be "were."
By the way, Maxsims, what is the function of “that” in that subordinate clause above?