I've heard the same caution about using synthetic in an 'older' engine, so I use Valvoline 10W40 motorcycle oil in my 1200 and my 1500 with the OEM filter.
Also, I have used Mobil 1 Racing motorcycle oil in my VTX since the 600 mile oil change.
Here is the reason they warn people that when they switch from "dino" oil to synthetic oil in an older built engine that it may leak oil at first....enjoy:.....sorry it is so long....
Back in the 70's and early 80's some synthetics were not blended correctly and caused engine oil leakage in some vehicles. Basically, the problem is that synthetic base stocks do not react the same way with seals and gaskets that petroleum oils do. PAO base stocks (the most common synthetic base stocks) tend to cause seal shrinkage. If the proper additives are not used, seals will shrink when using a PAO based oil, and leakage will occur.
Fortunately, oil manufacturers learned their lesson and reformulated their oil to contain the proper additive package which condition seals and gaskets to maintain their flexibility while also maintaining proper seal swell. In fact, the reformulation in most cases provides for better seal conditioning than most petroleum oils these days. Nevertheless, there is still a possibility of leakage if making the switch to synthetic - but only under certain conditions.
On older vehicles which have been lubricated with petroleum oils, seals and gaskets can begin to dry and crack. The reason you don't actually see leaks is because petroleum oils tend to burn off and leave sludge, grime and varnish on the inside of your engine. That's simply the nature of a petroleum oil's make-up. Therefore, some of the deposits end up around your seals and gaskets which actually plugs up the gaps which would have resulted in oil leaks.
The next obvious question is, "If synthetics have those special additives, why might they cause leaks in an older engine? Why don't they correct the problem?"
The "problem" is that there are also other additives that give a synthetic oil its detergent properties. In other words, there's other stuff in high quality synthetic oil which tends to clean out the sludge and deposits left behind by petroleum oils. Once these deposits are gone, the gaps around seals and gaskets become exposed and the oil might begin to leak in these areas.
It's likely that the additives will begin to lubricate the seals causing them to become more flexible and leading to seal swell which may plug those gaps over time. However there is no way of knowing how long it may take for this to occur or if the seals are already too far gone to be salvaged. If the problem doesn't correct itself, it is likely that the seals and gaskets would have to be replaced to prevent further oil leakage.
So, to wrap up. Petroleum oils can screw up your seals and gaskets and then fill the holes deposits to cover their tracks. Synthetic oils come in and begin to clean up the place. Once it's clean, the holes might be exposed, and the oil begins to leak. After a while the synthetic may be able to help the seals and gaskets regain their composure and stop the leaks.
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