I religiously change the oil on my Silverado 8.1 gas engine before 2000 miles, in the winter some point just after 1500 miles as that is when the oil will break down faster from cold starts and getting a richer fuel content.
And I always use Rotella T-6 full synthetic 5-40. During the winter months I do add one quart of Amsoil 5-20 and a half pint of Marvel Mystery oil to help starting when it gets down below -20F.
Are there any advantages to synthetic?