Ok so I have always ran synthetic oil in all my trucks. This is my first diesel so here is my question. Somone told me that with mobal1 and royal purples oils in my truck all I have to do is change the oil filter when my dic says Change oil and that the oil never had to he drained because the oil doesn't break down. Is this true? I have had my truck for just under 1 year and 35k miles and I'm about to switch from conventional oil to synthetic.