BSwope,
When I used the Amsoil HDD 15w-40, I used to change the oil every 10K miles. I could have gone the full maximum change interval (25K miles normal, 15K miles severe) recommended by Amsoil, however opted to change at 10K miles because that was the break-even point versus buying petroleum-based oil. And of course, I changed the oil filter every 5K miles (with Fleetguard).
Although, since changing to Amsoil Series 3000 5w-30 HDD oil, I plan to extend my oil change to 15K miles... that's if oil analysis proves I'm getting the protection promised from the use of this product. Once I deplete my stock of Fleetguard filters, I'll switch over to the Donaldson ELF7349 to coincide with oil change interval. Of course, I'll continue to perform oil analysis to confirm protection and continued use of oil. 15K miles of use on 5w-30 will be the break-even point for this product.
I don't tow heavy so this maintenance schedule, although more often than recommended, works for me and doesn't exceed the cost of using a good petroleum-based diesel oil. Getting the oil at dealer price makes this all possible.
Dwayne