Synthetic Oil Viscosity Ratings - More Confusion?

["Michael Stocker" <mdstockr@neo.lrun.com>]

Was checking out Mobil 1's web site at
http://www.mobil.com/consumer/mobil1/mobil1/mobil1pds.html and came across this
statement:

"Mobil 1 Formula 0W-30, Mobil 1 Formula 5W-30 and Mobil 1 Formula 10W-30 can be
used anywhere an SAE 0W-30, 5W-30, 10W-30, 10W-40, 5W, 10W, 20W, 20, or 30
grade viscosity is recommended. Mobil 1 Formula 15W-50 can be used where SAE
15W-40, 15W-50, 20W-50, 15W, 20W, 20, 30, 40, or 50 is recommended."

Based on this, I can use their 10W-30 in my '89 90Q year round.

I wonder why they are offering all these grades, then?  Why not just 2 grades?

Michael Stocker 
mdstockr@neo.lrun.com 
http://home.neo.lrun.com/pim/home.htm 
PO Box 256, Canal Fulton, Ohio 44614-0256, USA