Re: Which one should i use:5w-30 or 10w-30?
Castrol Syn is not true synthetic, but it is pretty good. Get Mobil, or Amzoil for a truly good synthetic oil. True synthetics are olefin based and do not react with blow by gases and condensation the way regular oil does and therefore it stays clean and works well for much longer intervals.
I have used Mobil and Amzoil for years in various Mopars changing at 20-25,000 mile , or one year which ever comes first . I use regular off the shelf oil filters (motorcraft, Molil, Amzoil, NAPA) and change them 4 times a year. With each new filter I put in enough oil to top it off again. I never have to add oil inbetween filter changes. So at the end of the year I have gone through 9 quarts of oil. 5 quarts when I do the complete change and one more each time I change the filter. All of the cars I do this to have run great, gotten good gas mileage, and they look good inside. When you do change the oil after a year you will be shocked at what you see. The oil will still feel good between your fingers, it pours out of the engine with no noticable thickening, and it doesn't smell to b ad either. An added bonus is this. It is cheaper to do this than use conventioal oil changing at 3,000 mile intervals and you don't have so much old oil to get rid of.