Correct me if I'm wrong, but from past research, synthetic oil has a different characteristic than conventional oil. The compounds/molecules in synthetic will outperform dino oil in extreme hot & cold operating temperatures and won't breakdown as easily. Thus, it will outlast dino oil. It makes sense as the 4R uses syn diff oil which will last for a long time unless you're towing, etc. In reference to the sludge issue, I don't believe it is true for all Toyota engines, just certain ones. I knew about the sludge problem before buying my '04 4R. If Toy had a sludge problem with the 4R, then why would they recommend a 5k OCI instead of 3750 or 3.5k? My mechanic's wife drives a '99 Sienna and used to change the oil every 3-3.5k miles, but when the sludge problem/TSB was discovered, they changed the OCI to 2.5k miles.
BTW: I LOVE my '04 4R and I plan on keeping her for 15+ years and well over 300k miles. I always make sure every service is performed and done correctly. If I had only plan on keeping her for 6-8 years, I wouldn't bother w/syn oil, etc. My other vehicle is a '93 Nissan Quest (yeah, a minivan, but I use it mainly for work) which was handed down to me by my parents w/20k on the odo. Now she has about 165k miles and still purrs like new. I'm the kind of person that notices every ding, scratch, sputter, and abnormal sound & vibration in my vehicles. It's nice having a mechanic who you can trust to do the repairs. I take my Quest in every time there is anything remotely wrong w/her. Bad thing is that there's no comparison when it comes to resale value. My friend's (mechanic's son) '95 4R, which cost $500 less than my '93 Quest when new, is worth so much more now if we both were to sell our vehicles. Nissan is good, but Toyota is better, IMO.