The reason each manufacturer has a slightly different taper on the glowplug interface (only for turbo glowplugs of course, not normal ones) is because the thinnest part of the taper is *expected* to deform slightly from the pressure of the glowplug being tightened into place. This produces a tight seal. Each manufacturer uses their favorite aluminum alloy for the head button, so each manufacturer adjusts the exact angle of the taper to account for whatever their computer model says will happen to the head button when the glowplug is installed. (if you don't think a turbo head button deforms slightly when the glowplug is installed, try removing the head button without removing the plug first -- at least in my Picco engines, the head button is jammed tight in the sleeve when the plug is installed, but is easy to remove when the plug is removed first.)

I definitely haven't used every brand of engine available, so I suppose it's possible some manufacturers make their head-button tapers different enough that you really can't use certain brands of plugs with them, but the guys at my shop have never heard of such a thing. (at least one of them used to race a couple times a week for years, so I tend to defer to him regarding touring-car advice.) I suspect the vast majority of actual damage is caused by people over-torquing their turbo glowplugs because they don't bottom-out quite as solidly as normal glowplugs do, and since engines that use turbo plugs tend to be a lot more expensive, people chalk it up to "proprietary design" and are paranoid thereafter because repair parts cost so much.

Anyway, I use O'Donnell plugs in all my engines, including my Picco engines. They work like a charm and I can see varnish buildup only on the face of the glowplug, not at all on the taper, so nothing is leaking past the seal.