A 5W-30 is not the same as 30W. You can pour it hot or cold to see it's a different weight.
Most thin oils will not protect a lawnmowers high revving, workhorse engine like the heavier weights. You will burn more too. Just because the thin oils of today are built better for protection, so are the thicker oils that seem to work better in high-output motors.
The heavier weights were/are recommended for lawmowers more-so. The thin ones are listed in your newer lawnmower manuals because lawnmower manufacturers know repeat customer-ship is low & most lawnmowers sold are cheap ones... therefore they don't care about longetivity like a car manufacturer. They may even be under attack by vehicle oil companies to promote theirs in these manuals. So to keep them happy & quiet, the lawnmower-makers include the thin oils.
If you are getting 20 years out of your lawnmower using 5W-30, then you will likely get around 40 years service with a 15W-40, 15W-50, 20W-50... 30W.. depending on your climate. If I lived in northern Canada, the lowest #s I would go is 10W-40 in my lawnmowers. If I lived in Texas, I would use 20W-50 or 30W.
[ May 07, 2005, 02:30 PM: Message edited by: Triple_Se7en ]