I bought my Rubicon about 6 months ago been doing a bunch of work to it the oil looked clean so I waited 3000 miles, and changed it with some Mobile 1 I had 5-20 in it per what the oil fill cap says. When I was out wheeling last week we stopped at a BLM sign I got out to read it and left the engine running heard a rod knock. I only hear the knocking noise after it warms up so it could be piston slap which isn't always a big deal. I know that rod knocking was not there before. Maybe I should have left this engine on Dyno oil. Its got about 78k on it but it might be little high because the old owner didn't calibrate the speedo correctly and it is reading about 5 mph faster then you are really going I corrected it with the tuner he gave me. Maybe Ill try a heavier oil 5-20 seems light for Arizona heat.

I think 5-20 is too light and you will find lots of information online about not using that weight in the 3.8 even though it's recommended. I run 10-30 In mine and it has stopped all of the cold start engine noise.

The 5-20 is recommended to get better MPG in the 3.8. Even though the block is a 25 year old design that was intended for 10-30. Lots of posts on this subject, go do some searching....

I had the same problem in my 07. I switched to 5-30 and the knock pretty much went away. The engine also went from losing a quart every 1k to a quart every 2k.

mine had a rattle like a spray can, burnt 1.5quart 5W20 oil every 800-1k miles untill i switched to quakerstate 5w30 dino. no more oil burn(execpt like 1/4 quart at 2.6K miles)
my '07 Squirrel powered 3.8 loves the heavier lube

had 50K in my old JK used penziol platunum and or Mobile 1 depending on what was available at the store 5W20 and never had issues.

Use Mobile 1 in my SRT8 and have never had issues with that either?

Been running the 5w30 for over 77,000 and no problems at all.

Ill try moving up to 10/30 a friend says he only uses 15/50 Mobile 1 I used to use that in older cars I had. I wonder how much of a difference it will be. I just wondered if there were any rod knock issues with these motors.