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Synthetic fluids/oils are all the same size molecules. Naturally occurring or refined are not. As a result, over time, non-synthetic oils can fill in compromised areas of gaskets etc. If you switch to a synthetic fluid, it will typically work out these gunked up areas that are effectively serving as a gasket sealer. What is really happening, a problem is being exposed, the gasket needs replaced, but many people choose to take the position that the synthetic oil/fluid caused the leak, when in fact all it did was expose a problem. This is typically the reason why people will say they won't use it in older cars.
The problem with recommendations for oils is that everyone has an opinion. Here is an interesting view by a seller of gear products. https://forums.fordthunderbirdforum.com/threads/1966-synthetic-differential-oil.14787/ Since this is on older car, and you are unlikely to be driving this many thousands of miles a year, I would stick with conventional fluid. Each fluid has it's pros and cons. In the end, do what you think is best but read the opinions of those in the business.
