Oil in the pasta water floats, which is the whole problem

There is a short answer about adding oil to pasta water and a useful one, and they are not the same. What follows is the useful one.

What the tip claims

The advice is to pour a spoonful of oil into the pan so the pasta does not stick together, and it appears in a great many cookbooks. The reasoning is intuitive, since oil is what people use to stop things sticking in every other context in the kitchen.

It fails on a physical detail that anyone can watch happen, which is that oil and water do not mix. The oil sits in a layer on the surface, where the pasta mostly is not, and stays there for the whole cooking time. Vigorous boiling breaks it into droplets that recombine, which does not change where the oil ends up.

When sticking actually happens

Dried pasta releases starch rapidly in the first minute or two, when the surface hydrates and becomes sticky before it firms up. Pieces resting against each other during that window fuse, and once fused they generally stay that way. Stirring during those first minutes is the entire solution, because it keeps the surfaces apart until they are no longer tacky.

Using enough water helps for the same reason, by giving the pieces room and keeping the starch concentration lower. A pan that is too small produces sticking regardless of what has been added to the water.

The damage on the way out

When the pan is drained, everything in it passes through the surface layer where the oil has been sitting. The pasta collects that oil on its surface at exactly the moment it is hottest and most receptive. A film of oil is precisely what prevents a water-based sauce from clinging, which is the opposite of what anyone wants.

This is why oiled pasta often ends up with sauce pooling at the bottom of the bowl rather than coating the strands. The tip therefore does nothing useful while cooking and something actively unhelpful at the end.

Where the habit probably comes from

It works in a different situation, namely tossing cooked pasta in oil to stop it clumping while it waits. That is standard practice in commercial kitchens preparing pasta in advance, and it is entirely sensible there. The technique appears to have migrated from that context into the cooking water, where the same reasoning does not apply.

Checked against the record, it may also come from the observation that oil suppresses the foam that causes a starchy pan to boil over.

That effect is real, which gives the habit a genuine benefit unrelated to sticking.

Managing the boil-over instead

Starch foam rises because bubbles are stabilised by dissolved starch, and it climbs the pan quickly once it starts. A wooden spoon laid across the pan destabilises the foam where it touches, which buys some time.

Trace it back and reducing the heat slightly once the water is at a rolling boil is more reliable and costs nothing. Using a larger pan with more headroom removes the problem entirely and helps with sticking at the same time. If oil is used for this purpose it is worth knowing that is what it is for.

A claim being badly sourced does not make its opposite true.

What to do with the starchy water

The cooking water is a useful ingredient, because dissolved starch helps a sauce emulsify and cling. Reserving a cup before draining is the single most effective step for making a sauce coat the pasta properly.

Where the story actually begins, adding it gradually while tossing lets the starch and fat combine into something that holds together. Oil in the water works against this, because it interferes with the emulsion the starch is trying to form. Removing one habit and adding another turns out to fix the problem the first habit was meant to solve.

The takeaway

The oil never reaches the pasta while it matters and reaches all of it at the moment it hurts.

Ask who gained from the claim being repeated. That usually explains the rest.