Spendeth it up - literally, swalloweth it. The wise man keeps a store in reserve. He gains uprightly, spends moderately, never exhausts himself. But the proverb may have also a higher application. The wise man stores up all "treasure to be desired" of wisdom, all "oil" of divine influence, which strengthens and refreshes, and so is ready at all times for the work to which the Master calls him. Compare Matthew 25:1-13.

[There is] a treasure to be desired and (i) oil in the dwelling of the wise; but a foolish man spendeth it up.
(i) Meaning, abundance of all things.

There is a treasure to be desired,.... Gold, silver, jewels, and precious stones; all sorts food, as Aben Ezra explains it, and rich and costly, raiment; all which may be lawfully desired and sought after, and, when obtained, laid up for future use; which may be spared for their own service and that of posterity: but there are riches of grace, a pearl of great price and treasure in heaven, more desirable than these, Matthew 6:19;
and oil in the dwelling of the wise; which is particularly mentioned, because a principal blessing of the land of Canaan; much used for food, and was for delight and refreshment: and something of this was in the house of every wise, provident, and industrious man, for the use of him and his family; even though he lived but in a "cottage", as the word (f) signifies this is an emblem of the grace of God, which is sometimes compared to oil; which a wise man is chiefly concerned, that it may be in his heart, in his house, and in his family;
but a foolish man spendeth it up; the oil; he swallows it up at once, as soon as he has got it, and wastes and lavishes away what his wise father had provided for him. This may refer not to oil only, but to the desired treasure, wealth, riches, substance of every sort, he is heir to and becomes possessed of; and which, in a spiritual sense, may be applied to a foolish wicked man, who misspends his time, neglects the means of grace, and all opportunities by which men grow rich and wise in spiritual things; see Matthew 25:1.
(f) "tuguirolum", Mercerus, Gejerus.

The plenty obtained by prudence, industry, and frugality, is desirable. But the foolish misspend what they have upon their lusts.

The wise, by diligence and care, lay up and increase wealth, while fools
spend--literally, "swallow it up," greedily.

20 Precious treasure and oil are in the dwelling of the wise;
And a fool of a man squanders it.
The wise spares, the fool squanders; and if the latter enters on the inheritance which the former with trouble and care collected, it is soon devoured. The combination אוצר נחמד ושׁמן [desirable treasure and oil] has something inconcinnate, wherefore the accentuation places אוצר by itself by Mehuppach Legarmeh; but it is not to be translated "a treasure of that which is precious, and oil," since it is punctuated אוצר, and not אוצר; and besides, in that case מחמדּים would have been used instead of נחמד. Thus by אוצר נחמד, a desirable and splendid capital in gold and things of value (Isaiah 23:18; Psalm 19:11); and by שׁמן, mentioned by way of example, stores in kitchen and cellar are to be thought of, which serve him who lives luxuriously, and afford noble hospitality, - a fool of a man (כּסיל אדם, as at Proverbs 15:20), who finds this, devours it, i.e., quickly goes through it, makes, in short, a tabula rasa of it; cf. בּלע, Isaiah 28:4, with בּלּע, 2-Samuel 20:26, and Proverbs 19:28. The suffix of יבלּענּוּ refers back to אוצר as the main idea, or distributively also both to the treasure and the oil. The lxx (θησαυρὸς ἐπιθυμητὸς) ἀναπαύσεται ἐπὶ στόματος σοφοῦ, i.e., ישׁכן בפה חכם, according to which Hitzig corrects; but the fool, he who swallows down "the precious treasure with a wise mouth," is a being we can scarcely conceive of. His taste is not at all bad; why then a fool? Is it perhaps because he takes more in than he can at one time digest? The reading of the lxx is corrected by 20b.

The wise - Wise men lay up desirable treasures for their own use, and for their families. Oil is particularly mentioned, because that was a considerable part of their wealth and treasures in those countries.