All leather will stretch
As someone has already said DO NOT OIL them, clean them as normal with saddle soap and grease them to keep them in good condition
When you buy a pair of leathers make sure they are a pair, leather is cut from a hide ( or butt ) from the backbone to the belly , the backbone will be stronger and less prone to stretching. A problem with uneven stretching can be caused by one of your leathers coming from the backbone and one from the belly. If the leathers are made properly there should be a number at the point end on the inside - the number is punched in pairs when the leathers are cut , this makes sure that the leathers are a pair when they leave the workshop , so look for a pair that have the same number - they should have been cut from the same place on the hide and therefore should stretch the same.
The other things that are important is how the leathers are made.
Stirrup leathers should always be hand stitched, they should also be made with the grain ( or shiney ) side on the inside , many people think they have been made inside out but this is correct. The grain side is harder wearing than the flesh side - so it is the grain side that you want to come into contact with the stirrup iron and the stirrup bar of the saddle
Sorry for the book