How to make spaghetti stick together

Yes, it is true that we don't add oil to the boiling water. I'm not aware of any good reason to waste extra-virgin olive oil that way! Some oil is always added at the end, over the sauce, when the pasta is already in the plate! It has to be raw, so that it retains its fruit nuances and texture.

If you are doing cold pasta salad and want to avoid sticky pasta, adding oil in advance won't help. You have instead to wash pasta under cold water to wash the starch away (you can put the pasta in the colander directly under the running cold tap).

There is a legal difference: according to Italian law, pasta (which includes spaghetti) is made only with durum wheat. Other countries also accept soft wheat. The difference is in the amount of protein (mostly gluten): durum wheat has more, which in turn gives the pasta a somewhat snappier bite. Also, the cooked pasta stays eatable (as in, non-mushy) for a longer time.

Moreover, some Italian pastas are extruded through traditional bronze dies that leave a slightly rougher surface that makes for better sauce adhesion.

Avoid by all means all stupid Italian pastas made for tourists. Avoid stupid novelty shapes and (most) colored pastas. Some good brands with global distribution that will not let you down: Barilla, Agnesi, Garofalo, De Cecco (thanks for the tip).