When we were in school, my sister poured about a half gallon of used motor oil that was in an antifreeze jug in the radiator of her car and ran it, Made a hell of a mess. Lol
That's a big track, I remember getting one for Christmas that was slotless called TCR (total control racing) you could change lanes and pass other cars. With in three days we had wore holes in the skid pads on the bottom of the cars from nonstop playing.
My thoughts are check the line between pump and tank. Like said above sounds like having a cracked straw, sucking in air some where. Hopefully it's not in the pickup tube in tank.