I cannot afford an original HK91 or FAL. The last FN FAL's were imported into this country in the 70's, adjusted for inflation, they cost $3,000!
Which is a clue why the FAL went out of service.
DSA FAL's are excellent and so are the PTR91 rifles. Both have factory warranties.
All delayed blow back machine guns require case lubrication or they won't work. As said by Col Chin in his Machine Gun series:
Prior to WW2 there were a number of fielded light machine guns that had oilers. The Italians and Japanese with the Breda and Nambu's are examples.
However the Germans captured a Russian machine gun with a fluted chamber, used that in their roller bolts, and that was the end of oilers.
All roller bolt actions have these chamber flutes to break the friction between the case and the chamber. Only the case head provides the gas seal.
Original HK 91's apparently have deep flutes. If you search old posts you find people complaining about the first PTR91's damaging their brass, so PTR changed to less deep flutes. Now people are complaining about the flutes not being deep enough for their crappy surplus ammunition. PTR cannot win one way or another.
If you want a roller bolt to function you must use ammunition without tar sealant, which gums up the flutes, and you must keep the chamber clean.
Since the HK91 series of rifles are still in production, still used as a front line service rifle by a number of countries, that should tell you something about it being a decent, reliable weapon.