As a general rule of thumb for domainers looking to resell the domain, if both are the same price, the singular is usually the preferred .. obviously there are many exceptions and it really depends on the specific word(s) in question.
Singular is generally looked for by companies looking for a brand. As such these will generally be wanted by buyers with more money to spend.
Plural is usually used for listing/directory type sites. As such these will generally be wanted by buyers with less money to invest because their businesses are usually smaller than a company geared more towards creating products or selling services.
But note that usually both domains will not be available for the same price. If you can grab the plural for a fraction of the price of the singular then that could be the better "value". Depending on the key word you might also be able to sell the plural to someone who wants to use it as a directory quicker (although at a lower price).
Also note that singular brandables are heavily looked for by domainers with big budgets, so while the dollar amount they can make in profit for the singular can be significantly higher than the plural, it could very easily be the reverse that the plural is more profitable in terms of percentage because the original price you bought it for should have been significantly lower.
At the end of the day you really need to look at the specific word in question combined with your cost to purchase the domain. Because even if the singular is "better", it might not be the better value.
Beyond that however, there are more words that are completely worthless in plural than in singular .. again .. it really depends on the names .. there are cases where I'd take a singular in a non-.com before the plural in .com.
Another tip is to type the word into google .. as you get close to finishing typing the word how does google auto-complete the word ... generally if it comes out singular then you'd want to get the singular first .. but if it comes out plural, then there's a higher chance than usual the plural might be better (or maybe simply not as bad compared to the singular as the average word)
In the examples you gave above, all the singular would be better. With fuel/fuels having a very large difference between them.
Fuel could be a great brand for a sports drink, granola bar, anything related to energy, or even an advertising/marketing company wanting to give the impression they can add fuel to the fire of a client's campaign. PLUS .. it can also be about fuel.
Fuels is pretty much limited to an information or listing site on the topics of fuels (and even then people would only go to a site called fuels if they were looking for information on multiple types or to compare .. it wouldn't even make sense if it was for a company with only one type of fuel).
Take a look at the singular/plural of the specific word/term you're looking at and try to imagine what each could be used for. The more broad the possible usage, the better.