I have been teaching Physics at the introductory level now for over 30 years. In that time I have taught a lot of labs on friction. Using the small masses readily available in the lab and using motion detectors to measure the velocity of the objects neither I nor any of my students has ever measured heavier objects sliding down the ramp at the same speed as lighter objects. I know what the theory states. I am talking about laboratory observations. I also know that air drag can be significant for bob sleds, rolling cart, bicycles, etc., but I am talking about objects moving at very small speeds (significantly less than 1 m/s) so I don't believe that air resistance matters here. I have looked on line to see if there are any demonstrations that might give a clue as how to show what the theory says should happen, i.e. what conditions need to be met, but so far I have found none. I have seen demos where the two masses clearly arrive at different times (small difference but entirely noticeable) and heard the presenter say see no difference. So if anyone here has some real insight as what might be causing the lab masses to act differently than theory predicts i.e. the larger masses slide down faster than the lighter ones, I would welcome the insight.