As you might imagine selecting the correct lubricant and additives for a particular engine is a huge problem and especially one like the A-series that shares engine and gearbox.
One of the first problems is 'synthetic' oils can actually be mineral as many years ago the American High Courts decided that heavily refined mineral oils can be classed as synthetic !!!!! The best way to consider it is you simply get what you pay for with oils and if a 'synthetic' is cheap then its almost certainly not !
In terms of seals then Yes oils can contain an additive called 'Seal Conditioners' which "Swell the elastomeric engine seals to prevent fluid leakage." Obviously if you switch between oils with and without these then it can cause variations in the seals and probably leaks!
In terms of the best oil for the A-series then again its a big problem.....the engine would run fine on a quality synthetic such as a 10W30 or 5W30 BUT the gearbox probably wont !!! I am not a gearbox expert so I am not sure why this is so but I have spoken to many experienced engine builders who say anything less than a 50 grade causes premature gearbox wear !
Therefore you are actually selecting the oil to protect the gearbox and not the engine !!!!
I've heard very good things about Valvoline Racing 20/50 whic I'm sure you could get in the States so this may be a good option !
Personally I run Duckhams 20W/50 and change it not much over 5,000 miles !
Hope this helps !!