I also used Rotella dino oil in 15W40. I replace the oil once per year (usually have between 5000 and 7000 miles on the oil) so I cannot justify the extra expense of synthetic. Seems like a waste of money to me.
Most manufacturers recommend every 15000 miles or 1 year, whichever is sooner.
FMCA #F431170, GS #822128658, Escapees SKP #112655
2012 Airstream Mercedes Interstate Class B