Running the amsoil 12tbn 15w40 for about 7 yrs now in my 6nz.Run the cat extended life filter on motor and change every 30,000km,the amsoil bypass gets changed at 60,000kms.Oil gets changed twice a year, and average about 150,000miles a year.amsoil in tranny and rear ends also.I have a commercial account with amsoil i think i pay about $24 a gallon,there synthetic grease is really good also.I have had good luck with there stuff so it gets used in everything i own.everybody has there preference.

I was just about to place an order for amsoil 15w-40 but some other drivers I talked to here in WI said they were using the 3000 series 5w-30 and recommended that. I was going to add the bypass filter and use amsoil in the trans and diff. as well. Then I stopped at a couple of shops that I use and trust and asked them for their opinion. both were concerned that I might have troulble with leaks. They also said I have over 900,000 on my series 60 14L engine and it doesn't leak and doesn't use any oil between my 20,000 mile oil changes so if it ain't broke don't fix it.

My reason for wanting to do it was extended oil change, better winter starting, but mostly hoping for increased mpg (I'm at 7.3 right now). Maybe I should listen to my mechanics.

I wouldn't worry about leaks i think thats a myth,you can switch back and fourth between synthetic and dino oil and it wont do anything.that was one of the old synthetic myths.Detroits seem to leak in the same spots anyways diamond seal back corners of oil pan.I wouldnt worry, just go with what feels right.If you do go with amsoil make sure you get a commercial account with them,that way you get it at commercial cost.good luck