One of the coolest parts about the iconic race is the post-race festivities ... when the big winner chugs from a massive jug of dairy to commemorate the milestone.

The Indiana Dairy Association works alongside the race organizers to keep the tradition alive ... with three options to choose from -- whole, 2% and skim.

Arrow McLaren driver Alexander Rossi -- who came in first back in 2016 -- explained there's one option that trumps the others ... at least, in one significant category.

"You have to designate your milk," Rossi told us this week. "The majority of people choose whole because it looks better in pictures when you pour it over yourself and everyone else."

Rossi ain't lying ... the IDA released the results of a Milk Preference Poll held with the drivers -- and 26 selected whole, while only seven went with either skim or 2%.

As for the race, Rossi also explained what it's like to return to the scene of his huge victory in 2016 ... saying he's hit with the memories every time he comes back to the facility -- both the good and the bad.