Amsoil in mine at 65 hours, now at 310. Lots of trolling and harbor cruising, does not make oil. Annual OCI, 80-100 hours.
Not much difference in cost vs. yamalube. Since I have Amsoil in everything I own I don't question it, I just order out what I need and put it in.
In every oil test there are products that do as well as Amsoil, but never anything that is better. I don't extend drain intervals on anything. I have two allison transmissions, one in a duramx, one in a workhorse RV chassis with the 8.1, a bmw 3-series, 3 motorcycles, and a toyota. Motors, trans, and diffs all have synthetic. That's just how we roll. :wink:
The d-max got delo 400 last time, it only gets about 4K miles a year, and costco had it cheap. I did lose 1 mpg or so. Shell Rotella is also a good oil, but they don't have a marine rated product. If not for Amsoil, mine would get yamalube. No cheap stuff for these expensive motors.
BTW, oem oil filters only - the off-brands are crap.