I have a fielder wagon 141. doing 255k and ready for an oil change. currently using np 10w30 (10000km). ready for an oil change and was recomended to use amsoil/mobil 1 20w50. should i stick with 10w30, move up a bit to 10w40 or is 20w50 appropriate? do a lot of driving, particularly highway. any suggestions on grade and brand?