Chains don't stretch - they wear! A new chain has variation just from the tolerances on all the pins, links and holes. All the little high and dry spots accumulate their wear and you have your "stretch'! The pins have high pressure contact and the chordal action of the links around the smaller counter shaft sprocket is very ugly motion. Think about the geometry, change of direction and accel/decel involved. Chains are amazing that they last at all when you start pumping HP into them and make them turn sharp corners!
You have to do your test rotation with a repeatable applied load not by hand spinning. Attach a weight on a small peg taped to the tire so the weight falls off at the same point each time. Average at least 3X!
No o-rings, Dri-Slide (moly-based) to penetrate inside
all the clearances - first time let the carrier fluid evaporate overnight - clean, blow dry and re-lube after each run at B'ville! I first bought Dri-Slide when the guy was selling it out of the trunk of his car! It would free up clutch and brake cables that were rusted fast!
Works very well in your weapons, too!