A friend of mine just traded a RAV4 in on a new Grand Cherokee. He has $200 built up on his Toyota rewards credit card that they said he could use in their parts dept. He was considering stocking up on oil to use in his Jeep but the local Toyota dealer only carries Toyota 0W-20 in full-synthetic (he is strictly a synthetic user, won't even consider dino.) Does anyone see a problem with him using 0W-20 in his Grand Cherokee (spec'd for 5W-20)? I told him get onto some user forums and he might find others have done it without issues or even that Jeep had back-spec'd it, but I thought someone here might know off-hand.