SFP? it's standard now isn't it (well free so everyone takes it as an option?)
Otherwise Dunno? Oleo is oil/Air (Nitrogen) so the air is the spring and the oil is both the damping and quantity determines rising rate to the spring. During line servicing, they can't really account for oil level as this is a shop thing, so will measure extension and pressurize accordingly.
Thus as the legs "age" (or get towards shop overhaul point) the oil level lowers. Thus at any given extension, the correct pressure "adjusted" by line crew will give less of a rising rate than a new leg. Thus it will sink in a bit softer and feel better. Most types I have flown show a tendency to land smoother as the legs get towards the end of their time... my last type must have been a 5,000hr limit, as anything just over a 5,000hr total landed like a brick..
Not noticed as much on a 737 except for the new ones are screwed together a bit tighter so rattle less on impact.