If the owners manual states it’s O.K. to use 10W-40 as well as 10W-30, then using 10W-40 would be perfectly all right. If it only specifies 10W-30, I would resort to 10W-40 only if the car burns “a lot” of oil. The problem is deciding how much is “a lot.” If it uses 1 quart per 3,000 miles, most people would agree that it is not a lot at all. I personally think using 1 quart in 500 miles would be a lot. The point is that if the internal engine parts are so worn out that oil is leaking through all kinds of gaps, then it’s time to use thicker oil. If not, then stick with what the manufacturer recommends. Also, check oil often. Find out how many miles it takes to use 1/2 quart and add that much to bring it up to full (but never more than full). Don’t wait to go down by 1 quart to add.