Another Update: I installed the new fill valve from Kohler and all is well again. Only difference is the the factory one had a yellow top while this one is red. I It also came with different colored flow restrictors. The factory one was black so that's the one I used.
On a different note: Perhaps I'm just a more.... heavy duty user.... but does anyone notice that these low flow toilets leave more stuff stuck to the bowl? A single flick of the flush handle works fine for #1 but leaves streaks behind for #2. I find my #2 requires holding down the flush handle longer so I get a more forceful flush by emptying out the tank. This defeats the whole 'water saver' thing, at least when it comes to #2. If this is just the nature of the beast with these low flow toilets, I'm fine with it. In a given day, toilets are mainly used for #1, where only 1.28 gpf are being used.
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I think I had some sort of link to that earlier. So that's the one Kohler sent to you to deal with the leaky fill valve? They're all pretty much universal for almost any toilet (save maybe low profiles ones where it can't shorten enough), but just meant for product differentiation.
This is yet another variation on the Fluidmaster 400. I've seen one sold as an OEM Kohler part with a green cap. The float is 100% identical on anything that looks like it. It probably says "Fluidmaster/400 MODEL/ANTI-SIPHON" on the float. I just replaced my original 400A with a 400H PerforMAX. It fills a little bit faster, has this wide adjustment knob, and uses a dial that gates the bowl fill.
After taking out the original, I learned how to replace the seal. Your original problem could have been with a defective seal. The seal can get worn, or sometime it's just sand or other stuff messing with the seal, where it can be removed, cleaned, and placed back. It's a $3-4 part at a home improvement or hardware store. You need to make sure the lever is all the way up on the cap, and then the cap just twists off counteclockwise. Some of the replacement caps (with a new seal) are available. It's easier to do with with the lever separated from the adjustment screw.
Amazon has it for $1.60 right now:
Fluidmaster 242 Toilet Fill Valve Seal Replacement Part, Fits 400A Fill Valve - Faucet Parts And Attachments - Amazon.com
www.amazon.com
The bowl fill flow restrictors work differently than roller clamps or dial gates. The instructions for adjustment are to make sure the bowl is filled at its maximum (where any more water will just flow out the trap) and then mark the water line with some tape or a dry erase marker. Then flush and see if it fills up to the top. The bowl level is important for a better flush. If it doesn't fill up to the maximum, move to less restriction. That's easy with a dial, a little harder with a roller clamp, and only in discrete levels with those little restrictors that clip on. There may not really be a way to perfect adjust that for any specific toilet because how it works depends a lot on little things like water pressure. With an unrestricted tube, it will definitely get it to the top of the bowl fill line, but then it wastes a certain amount of water going straight into the trap.
Another thing you might consider (if you didn't already) is to change your connector line. After a while they can get corroded and the internal hose worn. Most have a stainless steel braid covering to protect the line, but it's usually just a plastic hose underneath that can go bad. Mine old one has corrosion including rust on the steel ends and a green copper corrosion on the brass nut. I just got a Fluidmaster Click Seal line, which has a torque wrench style click stop. I just kept on tightening it (seemed even too tight) until it just snapped and stopped at a maximum set torque. I thought I was going to crack the plastic shank on the fill valve, but apparently it was OK. There are different supply valve fittings too, so that has to be chosen correctly (I chose the 3/8" compression when I needed 1/2" FIP).
As for cleaning - that's just a problem with these low flow toilets. Even a 5 gallon flush might not do it much better. I had problems with waste sticking to the bowl in a hotel room that had an older toilet. Only a pressure assist is likely to help, and those things scare me. What actually helps is just letting it sit where the waste soaks in water, where it should flush away if it's soaked long enough.