I've owned 4 Motorhomes with the 454 engine and NONE of them used any oil in over 3000 mile changes. Where did YOU get that information? My currant coach has over 70,000 on it and it will go more than 3,000 miles and just be down a smidgen. I use Mobil 5000 dino oil in it, and change it yearly or 3000.in that case probably 5W30 oil and Dextron ATF for your 4L80E transmission, be aware some oil consumption on that engine is normal up to about a quart every 1,500 miles