Going to synthetic oil you might start to notice a little oil leak out of the join between the transmission and the engine bell housing. I believe the molecules are smaller and get past the seal easier.
Not sure how true this is but I put synthetic in mine and started noticing a small leak. I have gone to a semi synthetic to see what happens.
I have used synthetic oil all of my cars and trucks since 1977. I began using the fleet version of Mobil 1 as it was readily available at the distributor where my uncle bought lubes for his logging business. It was called Delvac 1 if memory serves. I basically believe that synthetic does not leak any more than conventional oil, however, I have had two engines that started life on conventional oil and were then switched to synthetic after a few miles (50,000 in the case of a 79 Chevette I got used) and 24,000 in the case of my LT). Both started to leak within 5,000 miles of the change-over. So, I admit that this makes me a little skeptical, particularly with engines of older design such as the LT.
I canít say that the synthetic caused either leak and it may have been purely coincidence. However, Tom Cutter, a fairly well known BMW tech told me that he has seen many BMWs that developed leaks on synthetics and had the leaks largely go away after switching back to conventional. I switched my LT back to conventional and the leaks did seem to slow down substantially, but my clutch was already trashed so it didnít matter.
I also tried Mobil 1 in my 87 Voyager and it caused the clutch to develop a shudder during engagement that never completely went away even when I switched back to dino oil.
I suspect the new seals I got from BMW are probably fine with synthetic lubes. They were a different color than the stock seals and I suspect probably a different material, but I have no way to no that. I am sticking with conventional Castrol oils in my LT. I donít ride much below 32 or above 100 so synthetics really offer little benefit in that range. And the claims of dramatic fuel mileage increases are pure fiction. Synthetics of the same viscosity as conventional oils will make at most a couple percent reduction in overall friction and that is pretty much too small to measure given the normal tank to tank variation in fuel mileage.
I suspect synthetic is fine in most LTs and I believe that JZ and others here have run Mobil 1 without issue, but given my experience and Tomís recommendation, I am staying with dino oil until the bike makers start shipping their bikes with synthetics in them. Although, I suspect any engine designed after 2000 will have no issue with synthetics.