Despite being dull, gray, and pretty unremarkable looking, and despite having a revolting smell on account of its coming out of the digestive system of a sperm whale, ambergris is a pretty sought-after commodity. Also known as “floating gold,” ambergris can fetch up to tens of thousands of dollars per kilo, not least because it is used in the production of highly expensive perfumes. Apparently, it allows scents to last longer.
Ambergris was also a highly sought-after commodity at the time when Moby Dick was written. The character of Stubb, the second mate, expresses his opinion that ambergris is worth a good deal more than oil. On the basis of his previous remarks, it would seem that Stubb is referring to whale oil rather than the black stuff that comes out of oil wells. Whale oil was quite a valuable commodity in its own right, though clearly not as valuable as ambergris.
In any case, Stubb thinks that it's well worth his while to join Captain Ahab in what others might reasonably regard as a pointlessly dangerous sea voyage. If he can get his hands on the valuable waste products of the great white whale, then he'll be a very happy sailor indeed.