The 3.1 and 3.4 will last as long as any other engine with proper maintenance. The biggest issue with them is lower intake and head gasket failure, and you can get replacement gaskets that are better quality than factory from Fel-Pro. I've seen several run well into the 290K range. That being said, they are far from my favorite engine. I'd rather have the 3800. To say they aren't long-lasting is an incorrect statement though. As far as oil, I'd say a 0W-20 or 5W-20 would be ok, although I'd probably run a 5W-30 conventional. Any synthetic is overkill for these. If you want to run it however, it's your car and money buying the oil, and I know from your post you are a synthetic user. If you decide to try the 0W-20, let us know how it goes.