I just wanted to chime in and say my rear main seal started leaking last year, and after changing my oil with conventional (three weeks ago) there are no more leaks. Keeping an eye on it, was surprised when my drip pan had zero drops of oil. I bought my TJ with 27k miles and I know the previous owner only used conventional. I had changed the oil on this about 4-5 times since, each time with synthetic. Engine was noisier but that was really the only downside apart from RMS starting to leak (didn't think it was related but maybe it was).
Anyhow, after doing some reading it seems the 4.0L prefers conventional. I know there is a lot of debate out there on which oil to use but if conventional means I don't have leaks, I'm sold.