I have a surbrban with a newly rebuilt 350. Its been broken in, i changed the oil at 100 miles, 500 miles and 1000, miles. I use the truck mostlty for towing a boat, anywhere from 30 to 400 miles a day, i drive about 8k to 10k a year. I have been usig a 5-30? What weight and or brand of oil should i be using?