Based on the information you provide, the suspected cause for this issue is higher than ususal friction in the lens' bayonet mount and in its aperture actuation mechanics. Possible reasons are:

• Damage, e.g. bent parts or corrosion
• Incorrect assembly
• Dirt has accumulated

Whether repair is required or cleaning and lubricating will solve the issue is impossible to say at this point. Further inspection which involves disassembly will be required.

Just “lubricating it”, in the sense of applying a drop of oil here or there, is not a proper repair procedure. It may cause other issue in the long run. Low friction between the fast moving parts of the aperture actuation mechanics is achieved by choice of materials and play in bearings, not by lubricants. Lubricants are only used for slow moving parts, e.g. the aperture control ring.

Given that good copies of this lens can be had for EUR 50 to EUR 100, professional repair is unlikely to be economic. If you want to give it a try yourself, plenty of video tutorials can be found online that show how the mount part is disassembled and what tools and skills are required.

EDIT

The picture that has been added shows some stains and residue. This may hint to some sort of liquid having entered the lens. Its residues may have made the mechanics “sticky”. Cleaning will very likely be required.

Whether cleaning can be successful depends on how far the contamination reaches into the lens and on your repair skills. Cleaning the mount assembly can be a delicate job because there are literally hundreds of small stell balls in the bearings. These are a nightmare to reinstall and opening the bearings is one of the last things you will want to try.

Overall, it may be easier to replace the entire mount assembly with a good one salvaged from a donor lens of the same type.