Complex sentence: TEL remains an ingredient of high-octane racing fuels, and of 100 octane aviation fuel, as a suitable replacement for it in the aviation industry has not yet been found.
Split into simpler sentences: TEL remains an ingredient of aviation gasoline and high-octane racing fuels. TEL is still in use today as a component of 100 octane aviation fuel, as a suitable replacement for it in the aviation industry has not yet been found.