Matt, what have you heard about the downsides of leaning the mixture?
Is there anything special about this "lean burn mode"? Would fooling the ECU via the O2 sensor be accomplishing the same thing?
I have no clue. Basically this is just like every other modification for gas mileage. It's full of supposed results and speculations. I personally think leaning out a car too much is bad, unless you have something else to replace the missing gasoline with (like hydrogen). I've even heard that using a K&N air filter results in a leaner mixture, which is why the MPG increases.
Honestly the last thing I want to do is take control away from my ECU. When I get my engine installed, it is designed and built for economy. the d15z1 engine is designed with the ability to go 20:1 or leaner AFR. Other engines are NOT built for that type of a mixture, so I'm not certain if leaning them out too much would be good or bad.
It's one of those things that people can talk about, but most people are afraid to do. I mean, who wants to risk damaging their engine? Not me. I guess any potential heat increases due to lean mixture could be overcome by using a super synthetic oil, but I don't know.
I've just bought a brand new obd0 pm5 ecu for my auto to manual swap, and now I'm thinking about seriously working on this. You can get a willem eprom burner, and burn chips for the damn thing. I have heard that it is possible to lean out the mph ranges where you would cruise (like 50 and 65 mph) in my case to something like 16-1 or 18-1 on my engine, which would be lean but not lean enough to cause lots of heat or damage, so this would be relatively easy as it is just altering the ecus fuel map to inject less at those speeds, but I think if I do the resistor mod I would have to read the AFR at all times for some testing before deciding how to do the ecu tuning, however, a good tune on the ecu might allow you to leave the IAT like it was and just create the same affect on the ecu, which is much better overall.