In general, the manuals for the 2000 Regal, 2002 Jeep and 1996 LHS are calling for 10W-30 preferred wt. If the Regal has a 3800, they have always preferred 10W-30 (88-90 3800s preferred 5/30). And as you stated, the jeeps are a proven design and like the 10W-30. I believe the LHS 3.5L has direct acting tappets, and the 10W-30 is preferred for hot days at idle and such for an extra measure of oil film..just like the Cadillac Northstar prefers 10W-30 (pre 01), for the same reason. I was told that when the engines were in the design phase, the 5W-30 oils were not as robust as they currently are, so for those reasons the 10W-30 was preferred in those engine designs. However, with the current quality of 5W-30 oils, it is said the engines will be just fine on it, with good maintenence practices of course. The jeep still takes to the 10W-30 the best, I would guess. The Regal either viscosity should be fine, too. All-in-all, 5W-30 or 10W-30 should be fine. I think it was recommended to just change viscosities with the seasons and/or temperature ranges for the specific OCI.
What was the quality of 5W-30 in 1985 or so, when GM and Chrysler were doing the designing of these engines?..albeit the start of the 3.8/3800 was already in production preferring 10W-30. I suppose, just like the older (proven) design of the jeep engine, the companies were using what worked best at that time.(?)