When we first bought our 2002 Nissan Xterra in 2012, the previous owner told us to always use Castrol on the vehicle. Ever since, I have been using GTX High Mileage 10w40. Its not a full synthetic, but its a semi synthetic blend. Now i've been wanting to try out Mobil 1 High Mileage and that was a full synthetic oil. Now i have heard that when you go from Synthetic back to conventional you can have some problems. GTX High Mileage isn't a full synthetic. Would i start seeing any problems if i go from GTX High Mileage over to Mobil 1 High Mileage, and then back to GTX High Mileage?