I've read where puck bottom paper filters might filter out the cholesterol raising molecule cafestol. Never heard of cafestol until reading about paper puck filters. Filters at the base of the puck might filter aromas and oils potentially changing mouthfeel as well.

If you're not into cleaning your showerscreen and the slot in the grouphead with the grouphead gasket it sounds like puck top filters help keep those areas clean. I got into the habit at the end of every session of wiping the screen and the slot clean of excess grind and oil transfers, flush scrubbing the showerscreen and doing an empty basket in the portafilter wiggle flush to finish. I like the idea of using baskets to keep oils and fines from getting sucked past the showerscreen but knowing that happens inspires me to pop the showerscreen and pull the gasket off to clean it more often.

Capuchin Monk wrote:It does filter out oil. "Paper filters are tightly woven and absorbent, filtering out most of the oils and micro-grounds. This is a good and a bad thing."

"most of the oils"?? How did they determine that?
And is it the same for espresso (under high pressure) as it is for pour over brews?

Searched for that discussion but didn't find anything specific about how extraction oils lubed up e61s.

For me oils and grinds cake up inside the showerscreen, in the grouphead channel and over the grouphead gasket. With e61 machines they gum up the backflush valve. If you don't clean them up often enough the buildup can mess with the extraction. Anything to keep the buildup down is a positive thing I think. DOW 111 is a better lubricant than extraction oils, IMHO.

I'll just say that I am quite skeptical of the idea that the e61 group (and specifically its cam lever parts) actually "need" any coffee oils from espresso extraction in order to function smoothly, or that using a puck screen has any negative effects on e61 function. One of the benefits I've noted since starting to use one is the reduced, if not totally eliminated, need to perform backflushing. BTW I almost always backflushed with detergent (JoeGlo) which of course strips away most of the oils accumulated in the e61 group, and if anything noted improved function after backflushing. I'm sure YMMV applies to this opinion, as with quite a few of the other opinions posted here on HB.com....

Seems to be anecdotal opinion that extraction oils should be preferred to keep e61 lever parts lubed up. My personal experience after a good long time with an e61 was after an aggressive cafiza backflush lots of dark residue flushed out for a while and my lever would go stiff. I imagine the lube and oils keeping the lever cam moving smooth washed out but that usually happened when the cam shaft gasket needed replacing and lever cams were wearing down.

The old school e61 is a manual spring, cams, gasket and washer kind of machine. Washers and gaskets over compress with time and get rock hard stiff, springs get weak, cams wear down. I maintained my machine like an old car that really needed an overhaul and by the time I got around to the overhaul most of the e61 parts were on life support.

My point is that I'm into taking stuff apart to keep it running well and never considered espresso oil deposits my friend. They gummed up the levers, blocked up the showerscreen and I was concerned about them going rancid and affecting the taste of my shots. Didn't make me take my e61 apart more often but when I did I cleaned all the oils and caked on stuff off, cleaned them really well with a cafiza soak and lubed them up with DOW 111. Great stuff DOW 111. Showescreen and gasket clean/replacement was done more often than backflush lever break down clean and lube. Cam lever break down clean and lube I should have done more often. Lots of YouTube tutorials and it's not hard if you're ok with hand tools.

If I had thought of top puck screening to filter out oils and keep fines getting into the group I probably would have tried it. Keeping those oils out, IMHO, is of greater benefit than the coincidental lubrication the oils might provide. Less backflushing with oil/lube stripping cleaners would have kept well lubed parts moving longer I think than relying on extraction oils.

But if extraction oils work for you have at it. There's no decisive better or worse AFAIK.

There's interesting discussions referring to the oils in the machine when trying to reproduce the espresso shots from good cafes. Where does that quality come from when it looks like they fast volume dose, barely tamp, lock in and pull. An intriguing theory is the machine with it's built up commercial volume extraction oils transfers a good flavour note to the extractions that would make extracting the same beans at home difficult to reproduce.