In the lower temps a 5W30 would definitely be thinner for starting and warmup, we run 5W30 in the winter in the subaru otherwise the lifters take a solid 5 minutes to quiet down. I wouldn't think you would actually be getting slap, there should still be enough pressure to keep tension on the chain, but it could be rattling against the tensioner a little bit, doubt it would hurt anything. I know that when I start mine up in the winter (running 10w30) it sounds like a diesel for the first 10 minutes of driving, but so far no problems.

86 4runner, pro comp AAL, Kayline Soft top, 31's but all stock