Look at page 19 (well, page 16 actually). The 30,000 mile service (page 19) is the same for both schedules, the 15,000 mile service (page 16) is different but for both schedule 1 (severe) and schedule 2 (normal) "reduction gear oil" is listed. 15,000 miles is a lot less than 90,000 miles.

In the end, the service writer was probably sputtering because he didn't know how to argue with alternative facts.