More pasta water for oil sauces

Yes, but the amount of pasta water you use is the main difference.

Pesto just needs a splash — maybe a tablespoon or two per serving. You’re not making a sauce; you’re loosening the basil paste so it coats the spaghetti without turning into a swimming pool. Too much water and pesto gets thin, oily, and sad.

Oil-based sauces (aglio e olio, garlic and chili, tomato-free stuff) need more. A quarter cup or so per serving. The starch in the pasta water emulsifies the oil into something that actually sticks to the noodles. Without enough water, you get greasy spaghetti swimming in oil. With