After Iridium, Meitnerium (Z=109)! But they can only be installed by a Quantum Mechanic, and so far are only available for Carnot engines!

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John

2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)

Click to see 87 300E