Loading closer to the gate, rather than along the back, will stretch it more, for the same force applied.
A hanger on a bolt in the wall is good for this (suspending the karabiner upside down from the narrow bit of the pear), and a maillon on a footloop to apply the force (you have to hold it in place to stop it slipping down while stepping in the footloop).
Jammed krabs aren't necessary a reason to discard, I find it amazing how tightly an inexperienced caver can do up their Stop attachment karabiner when they notice it's loose while their weight is on the rope!
White splodges of Aluminium hydroxide, grooves / pits from corrosion and wear in the 'hooked' load bearing section are far more sensible reasons to retire.
The screwgate threads aren't particularly finely engineered and shouldn't need greasing, I've always found that washing the mud out is more useful than lubrication.