It isn't the actual injectors that ice up but the fuel distributor manifold on top of the engine. I think that in the 1980 Model 421C they started to run an oil pipeline through this manifold and that cured the problem on that aircraft whenso modified. A lot of pilots don't read the POH about adding alcohol.
Yup.
GTSIO engines are realistically one of the most reliable opposed 6-cylinder piston engines TCM have ever produced, IF......you can keep pilots from ham-fisting the throttles on a regular basis.
I personally know one owner whom has run his GTSIO engines to 1800 hours, and still going.