So like buying the larger jar of jam that costs more than the smaller jar because the cost per ounce is less for the larger jar.

More like: (1) oil getting cheaper makes (2) cars possible which means (3) the average person can now go further to commute which means (4) they buy a car and get a higher paid job which mans (5) their oil consumption goes from "cooking and an oil lamp" to "transport to a job that pays better than the best you could get before automobiles displaced horses".

What would the food example be? Vanilla ice cream going from a rarity only the rich could afford, to a standard desert for all when the synthetic form was invented?

Not OP but yes if you consume more jam (not if you consume the same over a period of time, because you're then just paying less for jam over a wider time scale).

The example I've heard given is accounting and spreadsheets. It made accountancy cheaper, but people then started asking more questions and analysis became a thing.

Rather than just taking the reduced spend as profit, companies wound up increasing their accountancy spend overall.

Not exactly, unless you still wind up eating the larger jars at the same rate as the smaller ones.