Towards the mid 80s GM had a trio of FWD diesel cars with the same Olds 350 diesel and Turbo Hydramatic 4sp auto…The Cadillac Seville, Buick Riviera, and Olds Toronado.
Did the THM 325-4L have a lock-up torque converter?
It appears (wiki page) that the addition of an overdrive and the lockup torque converter is what added the 4L to the THM 325 in '82. The ill-fated diesel 350 cars were from '79 to '85, but my impression is that sales dropped rather precipitously after about '82 or so, so most of them came with the older non-OD (so 3 speed) and non-lockup transmissions.