I knew someone with a Challenger with the high performance 6.4 and it required 0-40wt oil, and had heard him mumbling about the cost. It was almost enough to keep me from buying a Ram 2500 with the 6 .4. I'm glad I did, there are places you can get high quality full synthetic for reasonable prices.
Walmart.com has both Mobil One and Pennzoil for about $25 for 5 qts.
https://www.walmart.com/ip/23636902?wmls...=sem#about-itemhttps://www.walmart.com/ip/Pennzoil-Platium-Euro-0w40-Sn-5-Quart/163165881If in doubt, and you don't know what viscosity to use? Check the oil filler cap and it will be clearly labeled.
