I belive that's only true on the 5spd. auto transmission using synthetic oils. on a regular tranny I'd use a Mobil-1 synth. transmission oil available at Autozone for cca. 5USD/quart ! check the manual, but I'm pretty sure the '91 auto tranny is a 4speed and needs an oil change at every 60k miles (30k maybe ???) also don't forget to change the filter...