Toyota will often keep a winning engine design going for decades, as we've seen with the 5-decade R engine. Toyota's inline-six M engine soldiered on for nearly 30 years.
Starting in 1965, when the crypto-luxurious Crown needed six-cylinder power, the overhead-cam M found a home in Crowns, Coronas, Supras, Cressidas, Soarers, Chasers, and- of course- the 2000GT. Displacement ranged from 2.0 to 3.0 liters, with both single- and twin-cam versions being manufactured. The last M-powered Toyotas were built in 1992.
[History Of Toyota's M Engine, Wikipedia]