I've been changing oil on my 2011 gas F350 at 7,000 miles. The last filter I bought (Purolator Boss) said 15,000 miles on it. Been using synthetic oil. I looked at the dipstick at 7,000 and the oil is not black, just medium brown and has plenty of body. I had read some people are getting 10,000 miles or more between changes. Don't have high miles on truck. Below 40K miles and no heavy towing or anything. I'm thinking of going to 10,000 miles for oil changes. What do you think?