As far as I know Auto-RX isn't snake oil.

Not that I use it, simply because the inside of my engine is clean.

Years of (cheap) dino oil use and long OCI's can leave varnish.
Usually on parts where the oil just flows over, but it will also be there (I can imagine) in oil lines, etc.
Some engine designs may get more varnish, like a V6 mounted east/west, the cylinder bank closest to the firewall gets a little hotter, the dino gets a little more cooked every day leaving varnish.

There are lots of pics on BITOG showing it.
And usually posters on BITOG recommend a couple of Auto-RX cycles to get some out.

Auto-RX has a solid reputation.

 

Auto-rx is popular because it is safer than traditional engine flushes, providing gentle cleaning without harsh solvents and using products that are beneficial to seals. I wouldn't recommend a traditional engine flush product.

Auto-rx has helped a few people who have suffered with recent Toyota sludge engines, it's about the safest way to tackle those problematic engines.

 

I have always consider varnish a cosmetic issue, nothing more. At 365,000 this thing is still running like a swiss watch.

 

I'm a big fan of Auto-Rx and I do use it in my S2000. I became a fan mostly because it saved me thousands of dollars in mechanical repairs with other vehicles I've owned. I didn't have a specific purpose in mind for its use in the S2000, so I can't point to anything that it has specifically accomplished so far with it. My philosophy about using ARX right now in the S2000 is to prevent bad things from happening that could and have happened to my vehicles in the past, even with completely anal preventive maintenance practices that are considered more orthodox.

For the S2000, I think that people experiencing high oil consumption and/or low compression issues could easily get some quick and long term help from this product if blow-by from ring coking is the problem. You might have to replace the PCV before and after treatment though. I've also wondered if it might help with TCT problems experienced by many here.

I haven't heard of very many F series engines developing varnish/sludge buildup yet, but ARX would keep this from happening.

I have seen carbon buildup on valves though, and a fuel additive would be called for to attack this problem. I have used Marvel Mystery Oil in the fuel successfully in the past to quiet valve-train noises on start-up, but MMO has some drawbacks. It's been shown to leave a film on the valves which might actually contribute to carbon buildup, and it has some ingredients that might diminish the life of the emissions system. No proof hear, but a VOA shows suspect ingredients that concern me. I've recently switched over to LCD, inc. FPPlus and it seems to do well with the noises on start-up, but you have to use 3-4X the recommended dose. There is nothing harmful for the emissions in FPPlus though.