I haven’t seen or heard anything about synthetic oil causing oil leaks, before.
Not saying it will do it – but logically, I don’t see how it can.
My opinion is: “synthetic oil is better than conventional oil – but” (and a big but) the reason we change oil - is to get rid of the contaminates. Contaminates that is too small for filter to catch, that is. I use the term “contaminates” generically and it means anything that didn’t come from the oil container, initially.
If there was a way to keep contaminates out of the engine oil, thereby allowing the oil to stay cleaner, then I’d be all over synthetic oil like “kids over ice cream”. On top of dirt, carbureted vehicles just put too much “fuel” in the oil, as well, when engine is cold.
I change the oil in everything in my fleet every April and October – regardless if the vehicle wants it or not (and lawn equipment every April). That is about an average of 5k miles per vehicle between changes.
Transmission and differentials – are close to the same. The reason to change fluids is to get rid of contaminates.
Transmissions (automatic and manual) as well as differentials get hot when in operation. All three items have external vents on them so as unit gets hot, built-up air pressure does not blow out a seal (or something). The excess "hot" air exits via the vent.
The problem is as unit cools down, outside air gets pulled into the same unit. This outside air could contain high humidity, dust or even bugs, sometimes. If a person could install vent air filters for each, then the fluid change intervals could change – but cost for such a filter and modification time, might not be worth the cost of fluid changes.
In the case of automatic transmissions, a person needs to adjust the bands about every 30k miles, anyway – so cost of synthetic ATF seems like a waste, to me.
NOTE: I am not saying not to use synthetic fluids – for there is no question in my mind they are superior to conventional fluids. On our cars, especially for those who have not upgraded to fuel injection yet, and thereby need engine oil change more frequency due to contaminates - the cost of synthetics do not appear to be cost effective. To me.
The newer vehicles – the cost disadvantage vs. fluid effectiveness – synthetics make a lot more sense.
NOTE: I will support anyone pushing for synthetics – AS WELL AS I will support pushing conventional fluids. This is not anything worth arguing or debating about, in my mind. I will save my energy to argue other issues which I feel more strongly about. The type or brand of fluid is not worth the effort to argue about.
Now well maybe the frequency of fluid changes and/or viscosity, might be worth the effort . . . maybe.
Most people I know of are already set in their ways on which type of fluids – except for those who are not knowledgeable on the subject and are asking for more details.
I’m not going to push either way, only present the facts, as I know them.
BudW