Could be any one of a number of problems. Since you bought the rifle in apparently unfired condition....it is possible that over the years, whatever oil or grease that was inside the action has hardened and the resulting gunk is affecting the operation of the lifter. As you mentioned, after you oiled it, the rifle ran a lot smoother. Still, a complete strip down and thorough cleaning might be in order. Then, see how it runs.
If that doesn't solve your feeding issue, the first thing I would check is the loading gate screw. Is it tight? If loose, this can cause feeding issues. Next, the guide rails. Are they tight? The lifter, is it stamped? Pre-64's were machined. Post-64 were stamped metal. If so, the lifter may be worn or bent downward slightly. You may need to bend it slightly upwards to correct. Or replace it altogether. It bears mentioning that some of these areas can be checked while you have the rifle stripped down.
Slowly open the action. At the point where the bolt pulls the top of the lifter back, does the lifter snap upward, or lazily follow the bolt up? With the bolt open, push down on the lifter, with a finger. You should feel some resistance....then it should snap down. If the lifter does not snap up or down....then something is loose, broken, or worn. In which case, you might have to replace the lifter. Could also be the lifter spring.
While you have the rifle stripped down, examine all the working parts, and the inside of the receiver. Do you see any areas where the parts may be binding? Rough machined areas on the working surfaces, or metal fragments? These might just need a good stoning or polishing to get them working smoothly.
This pretty much covers the basics. I suspect the rifle is just gummed up and needs a good cleaning. But, there could be an issue with the lifter.
While you're at it, work the carrier up and down. You should see the carrier spring screw, on the inside left side of the action....opposite the loading gate. Does the carrier spring move? If so, the spring screw may be loose and will require tightening. You'll have to remove the loading gate to tighten it.
Best case scenario: It is likely the rifle just needs a good strip down and thorough cleaning. Since you bought it apparently unfired....it is possible that there is a lot of old, hardened oil or other gunk that may be affecting the rifle's operation. Hope this helps.