I don't know if it is me or not, but after buying my wife's 2004 Jeep Grand Cherokee with 45,000 miles on it, I switched the usual stuff to the better Mobil 1 Full Synthetic Stuff (10W-30). After doing so, upon start up and every once and awhile now, I am getting some pinging noises. Why is this? I have the same problem with my Wrangler which I started at about 5 miles on it using the full Syn. Why do all my 4.0s ping when using the synthetics and should I just run cheap Regular oil? Any advice would help, because I really hate hearing that pinging noise every so often when I start up Does anyone else have this issue? I am now looking for a good motor oil. Thanks for any imputs or comments.

I don't know if it's your problem or not. Hard to tell by just a description and not hearing it personally. But, I do know that 4.0s are notorious for piston slap. My 95 Sahara has done it since it had a few thousand miles on it, and it has about 70K on it now. I also use Mobil 1 full syn. in it. I never did notice the relation to the oil though. My 91 YJ did it too. ?? I wouldn't worry too much about it though.

Also you should have broken the 4.0 with non-synthetic - for 3-7K and then
switched to synthetic...

The noise on startup could be the lifters. Cheap *** oil filters with poor anti drain-back valves allow the oil to drain out and it takes a bit for them to repressurize.

A good oil filter - and a quart of Lucas - would help that.

I have used the Castrol Syntec in my 94 XJ with the 4.0 and the 258 in my CJ. Both motors were broke in on dino oil then switched to the Syntec with the mobil 1 filters.

Never had any pinging on the 4.0 and it depends on where I get gas on the 258.