Engine Oil Discussion

The origin of many of the stories regarding seal failure and leakage was specific to the first synthetic motor oils on the market.
There are a number of resources you can investigate if you want more in depth of what is called a 'synthetic'.
IMO Richard Widman's paper is one of the best summaries in plain language.
Here's English version Classic_car_oil

Whether to go for a -30 or a -40 depends on what you observe during long drives. If when the engine is fully hot, it seems to burn more oil and perhaps runs a bit lower in oil pressure (if your model has a gage) at highway speed, then a -40 might help a little. Otherwise 10W-30. And if its leaking a lot, no point in spending a lot more money for some specialty oil.