Seller's mechanic says change synthetic every 3k miles. I won't be driving van like seller did...he put lots of miles on & most in city. I'll be doing long highway driving. Forgive me, but I already read thru thread & not sure how to proceed. 1. What weight synthetic to use & 2. what change interval for this T&C engine currently 116k? I'm a single woman over 70 & I really want this community's input because I'm afraid my status will lead to being gouged. TIA.