I was having issues with my extruder slipping on the filament and causing under-extrusion, which got even worse after I added an oiler to the filament. Instead of printing or buying a whole new adjustable extruder assembly, I decided to make a simple pre-tensioner to sit on top of the existing spring to tighten the grip on the filament. This may not be the perfect fix, but it was quick and it worked for me.
This adds 3mm of extra compression to the spring. If it needs to be shorter, just cut 1 or 2 mm of the bottom in your slicer.
I guess this will work for other extruders of similar design if they use the same size spring, but I only have experience with the MK10.
Looks nice at 0.1mm, but I think it comes out cleaner at 0.2mm because of the way the top prints. This may just be my printer setup though.