I certainly don't have your level experience with these engines, but it seems like if lead is an issue, "optimum mixture" may be too rich. It may be optimum for combustion, but still result in more lead fouling than needed. Since there is no harm in leaning a good bit beyond "optimum" at these low power settings, wouldn't that help prevent lead build-up?
In practice, some people just seem to have issues, and others don't. With my low-compression setup, I am fairly religious about leaning on the ground as well as in flight when able, and after almost 200 hours have never had a fouled plug or any other lead issues. Maybe what I am doing is working, maybe it wouldn't be an issue anyways, but it doesn't seem to hurt. I am open to more knowledge though!
Chris
First, the most common problem that people are looking to address with leaning on the ground is lead fouling. Unfortunately lead fouling has little to do with mixture, it really has to do with heat. Your fuel has scavanger additives that will help prevent lead from depositing by converting it's form (lead oxide -> lead bromide). Unfortunately, this conversion requires a certain temperature and you typically won't get to that combustion temp until around 1200 RPM. The best way to eliminate lead deposits is to never run your engine below that specific combustion temp (not that it is a realistic goal). Maintaining an optimum mixture will marginally reduce the potential for deposits, but don't kid yourself, the deposits form at pretty much the same level at most mixture levels. The deposits are not due to a rich mixture level. All of the fuel (rich, lean or otherwise) contains lead oxide and all of that lead oxide is available to deposit in the chamber unless it is converted to lead bromide (the lead oxide is not burned or otherwise changed during the combustion process like the hydrocarbons are - this is why they add scavengers). The mixture does nothing to accelerate or inhibit the potential for that lead to deposit. Only conversion to lead bromide can prevent the deposits. It does this by getting the combustion temp up high enough for hte scavangers to do their work
Sure, if you reduce the total fuel introduced to the combustion chamber by 5% by leaning from fitlhy rich to optimum, you can theoretically reduce deposits by 5%, but I can't imagine that helps.
Now, leaning can increase combustion temp, however, it is not moving it much at low RPMs. I have not done the research, but I doubt you could be precise enough in your leaning activities to make that much of a difference. Also, overly lean is just as cold as overly rich. The only thing that could help is to maximize combustion pressure (think red box), thereby increasing combustion temp. That said, anything that you can do to reduce the total fuel delivered and, more importantly, to get to the desired combustion temp isn't going to hurt. I am just not convinced it is helping much.
My goal here is not to encourage anybody to change their behavior, only to encourage people to stop honoring old wives tales and do the research necessary to truly understand what is happening in your engine and develop strategies to address any issues.
Lead deposits are a reality with 100LL. There are steps that can be taken to eliminate the build up. There is a reason that Lycoming recommends running your engine up to 1200+ RPM for 30 seconds before shut down. This helps to burn off the depoists that were built up during the low RPMs during taxi. It does this by getting the combustion temp up high enough for hte scavangers to do their work and convert the lead oxide. Spark plug type makes a big difference. The plug mentioned earlier in this thread was designed specifically to reduce deposits by making the crevices bigger and letting the deposits form in such a way that they won't bridge to the center electrode.
I would argue that some of these steps and other issues create the difference between user experiences with fouling and not the mixture setting. Again, just some food for thought and not a crtiicism on how other manage their engine.
Larry