There would be way fewer whales.
Plausibly. OTOH, given the stuff that tends to live in the deep wide oceans in D&D setting worlds, whaling seems less likely to be as big a thing as in the real world.
Unless the world is as magic heavy as the core books imply but few DMs run, it would be a considerable necessity as peat bogs and forests evaporate in places where they aren't full of PC fodder. Societies need fuel, plain and simple.
Was whale oil mostly used for lighting and as a lubricant?
I wonder if ruby mining would become the big thing for continual flame (50gp and you get a forever lantern that you can pass down as an heirloom) -- or if there would be a market for apprentice mages to wander the town for a few hours every night casting a ton of light cantrips until the families went to bed.I think the biggest reason that technology doesn't advance the same in a fantasy world is that there are alternatives to solve many problems - Magic!