Huh?????!!!!!!!!!

This was posted on NHTSA

Year : 1989
Make : FORD
Model : THUNDERBIRD Year : 1989
Service Bulletin Num : 99816
Date of Bulletin: MAY 03, 1999
NHTSA Item Number: 606420

Component: ENGINE AND ENGINE COOLING:ENGINE:GASOLINE

Summary:
FORD MOTOR COMPANY NOW RECOMMENDS SAE 5W-30 VISCOSITY GRADE FOR SERVICING ANY FORD GASOLINE-POWERED VEHICLE REGARDLESS OF MODEL YEAR.

My question why?

Do search for '89 here: http://www-odi.nhtsa.dot.gov/cars/problems/complain/ComplaintSearch.cfm