White deposits = lead oxide.
In 1921, researchers at General Motors discovered that adding a compound called tetraethyl lead to gasoline could improve engine performance. (Not-so-fun fact: Thomas Midgley Jr., a scientist who played a key role in what proved to be a calamitous discovery, also developed chlorofluorocarbons, a class of refrigerants that went on to damage the ozone layer.)
I’d like to see his epitaph!
Have you tried TCP (made by ALCOR, the EGT gauge manufacturer)?