From 5w30 to 10w40 + Lucas Oil Stabilizer = Rattling and knocking increased.
 

Dad's 2000 Dodge Caravan 3.0 Litre was rattling and knocking a little bit and using oil a bit excessively though not smoking out the tailpipe. Initially I was using a 5w30 and was topping off with 5w40 Diesel oil and also had a container of STP Oil Treatment in there. So, in attempt to make this rattling and knocking go away we thought to try thickening the oil. I tried finding a 5w40 within certain limits; didn't work. I could've used a Diesel oil and was concerned it would clean the engine too much and increase oil consumption even more, so I didn't use that. I could've used Mobil-1 0w40, it wasn't a High Mileage oil though. So we ended using Pennzoil Conventional High Mileage 10w40 and Lucas Oil Stabilizer; This made the engine go from knocking a little bit, to knocking a lot more. Even after letting the van stay on for over half an hour it still did it.

So, now I'm thinking about taking the oil I drained out of the crankcase and putting that back in. It still has at least 1,500 miles worth of use in it. We changed the filter at least.

Earlier, I changed Mom's van's oil from 5w30 or 10w30 & STP to Castrol Full Synthetic 0w20. The van wasn't rattling before and now I think the fuel economy went up.

EDIT: Somehow this noise stopped being so noisy today. It wasn't so clear in the first hour of the oil change though so take this with a grain of salt, would you?

As it turned out that knocking sound is the power steering pump pulley. The engine's fine.

Did you also type that on a typewriter?

No, why are you asking?

I don't understand some people's afflictions with overly heavy oil. Guess that's why I am a member at bob is the oil guy.

For what it's worth, a friend of mine has a 1990 Jeep 4.0. It plugged the oil pump and he drove it home with the bottom end knocking *and* the lifters clacking away. We cleaned it up and suddenly it leaks oil pretty bad now that the oil pump is actually pumping.

The engine was whisper quiet, though, after we fixed the oil pump. Until we dumped in 20w-50 to slow the leak. It sounded worse than my 2000 with either a broken piston or a bad lifter.

My Jeep 4.0 is actually a hair quieter on 5w-20 than anything else.

Reason why I wanted to try this is out of hoping it would stop using up oil as fast. On longer trips it certainly at least seemed to use oil a bit excessively.