I hope this isn't the first time you've changed your oil! ;-)
Advised by whom?
Unless you live in a very hot climate, there's no need to use a 15W oil. If you live in an area where it gets cold, you definitely DON'T want a
15W oil. A 10W oil is plenty for most conditions. If you look in the owner's manual, you'll find recommendations for oils for various environmental conditions. If in doubt, it's hard to go wrong with 10W-30 or 10W-40. Lighter oils such as 5W-30 are recommended for most newer engines.
As for semi-synthetic, that's up to you. It will provide better protection than a natural oil, but not as much as a full synthetic. Many people advise against switching to full synthetic in a car with high mileage on it, which may be why a semi-synthetic was recommended. However, 60K miles is not exactly "high mileage".
Natural oils work just fine as long as you change them at reasonable intervals. Again, the owner's manual will provide you with change interval recommendations that are based on the use of natural oil.
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