Do one-sided clip-in pedals have a bit more friction and inefficiency than alternatives? Moving from old style pedals years ago, to when I took up cycling again more recently, I noticed a difference when oiling the bike, while doing maintenance. 

After oiling an old style pedal, I would always flick it to see that it rotated more freely due to the new oil.  It would do several rotations and take a few seconds to slow down.  But doing the same with my newer clip-in pedals, they only do a few turns.  

Is this due to the one-sided pedal (Shimano LOOK pedal design) having a weighted end?  That is, does this pedal slow down more quickly after it is flipped, due to it's asymmetrical pedal weight distribution?  Or, do people think there would   inherently be more friction in such a pedal?
Likely two things.


Bearing seals will slow down free spinning. In actual use the added resistance from the seals is minimal and the seals ensure the bearings remained greased and in good working order despite the weather conditions (e.g., from dry and dusty, to wet and gritty).
SPD-SL are weighted so that the single sided pedal platform faces you (i.e., the rear of the pedal settles in the downward position), making engagement after a stop much easier.  Without the weighting the pedal would likely present either side (cleat platform or backside) when you start pedalling making consistent and easy cleat engagement a no go. Anything that is not evenly weighted will not spin as freely as something that is balanced (e.g., unbalanced loads in a washing machine). In use the uneven weighting isn't an issue as the pedals orientation remains static in use.


In short, the test you derived for old style pedals worked well for them, but hold less meaning for newer Shimano SPD-SL road pedals.