I’m in southern IA where we can get those cold winters..
Someone I know said not to use synthetic oil in a 242 since the engine was designed in 1962 and because the tolerances are greater, synthetic oil will cause leaks since it’s smaller on a molecular basis than conventional.
I'm curious when this old wives tale will stop being told.
Synthetic will do no harm to that engine. If you do some research in your free time, you will see countless articles written by people other than "Someone I know" that clarifies this is an old school mentality, that had some validity when synthetics were new and vehicle/gasket technology was old. Times have changed, as have designs and tolerances on engines and gaskets.
As mentioned above, your Jeep is from the 2000's, so you will be fine. The engine has very loose ties, if any at all to the old "tractor engine" that many claim the 4.0 to be.
All that being said, my 2000 XJ currently has 161,000 on it, been running synthetic 5w-30 since about 60k when the vehicle was given to us by the original owner. To this day, I only add about 1/2 qt during my 5,000 mile OCI.
The 4.0 is a pretty stout engine, if you put oil in it, and keep oil in it, it will outlive the vehicle around it. I like using synthetic for the cleaner properties, and the "extended" OCI of 5,000 miles I use.
Happy Jeeping!