Always change my own oil, don't trust anyone else. I also change weights summer/winter. When we're going to tow in hot weather thru the mountains I use Mobil 1 15-50. Winter, Mobil 1 5-30. Towing in Spring & fall may use a mixture of 2 qts 15-50 and 3 of 5-30 to protect engine when working it hard but try to keep some lighter weight oil for easier starting in the cold. The '94 GMC 350 has 150,000 on it so I'm careful to feed it what is best for it at the time. Of course a new oil filter each time.