This is an urban legend. I've added oil to the water for decades. After cooking and draining the pasta under cold running water, there isn't enough oil to make any difference whatsoever and I never have clumpy sticky noodles.Noooooo! Putting oil into your pasta water will coat the pasta with oil, which will prevent pasta sticking together, but will also prevent sauces from being absorbed by your pasta.
The reason to slightly undercook pasta is so you can finish cooking it in your sauce, and the not quite fully cooked pasta will soak up your sauce, kinda' like a sponge.
Mix the al dente pasta with your sauce, stir it around, and add a little bit of starchy pasta water. The starch will help the sauce bind to the pasta.
Meryl, please correct me if I got any of that wrong.
That said, please continue to cook in any way that makes you happy.
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