[andreareina]

Buttered noodles are good, I have no a priori reason that simple oil wouldn't be also.

[louistsi]

Butter emulsifies into a sauce just from the residual heat of the spaghetti (and some mechanical action - stirring, pan flip, etc)

Oil needs a bit more help, otherwise it's just grease on noodles. The starchy water the pasta was cooked in can do most of the heavy lifting there, but the addition of garlic helps too.