What type of fluid is used in the transmission of a '89 F150 w/standard 5 gear?

What are problems that could arise from using wrong fluid?

 

You have either a Mazda M5OD or ZF S5-42 5 Speed. The ID tag is on the driver side of the tranny. Both tranny's use Mercon, thats right Mercon. The Mazda capacity is 7.6 Qts and the S5 is 6.8 qts.
If you put in 80w you need to flush it with Mercon a couple times or so and hope for the best.

Had a buddy that did that, the tranny was working fine when he traded it in.

 

BTW, only 1992 and earlier 4 speeds used 80w.