I am putting about 28k on my car a year. I change the oil atlease every 4,000 miles . Thats 7 times a year times say 24.00 = 168.00 give or take a couple dollars. Would it be better to use a mobil synth. oil and change the oil every 8,000-10,000 miles? I think they charge under 50.00 to change the oil and put in synth. oil.? Is it worth it.