Right, there is ALOT of misleading info and half answers here.
Firstly, the choice of what oil your respective dealer puts in depends on what they buy in bulk, they do not stock a different oil for different vehicles. They have oil on tap hanging from the ceiling...so you get what they get in bulk. And its not like any of their techs know the first thing about oil anyway or what to use.
The correct oil you should be using depends on which side of the manufacture tolerances it was built, average ambeit temps, type of driving, peak oil temps and average oil temps, rpm etc etc.
However we will never tell what side of tolerances the engine was built with unless we take it apart and measure down to the crank journal, cylinder bore, rings etc etc.
Best you can do is take an educated guess. Suppose you break down into summer and winter grades you run rather than an all year round grade. SUmmer ambeint temps are higher so you can run a higher base viscosity, peak oil temps will also run higher so a higer high temp viscosity is better. FOr most a 15w40 or 10w40 etc etc. In winter a 5w40 etc.
Things not to worry about are theories of leaks with differing grades of oil. Modern engines do away with most gaskets and replace them with rubber based instant seals. These do not contract and expand, crack etc etc, so leak will not occur due to an oil change.
Also the debate over semi of fully synth oils. FUlly synth oils are better, end of. They are more thermally stable, have far better shear stability and can hold a film layer, they have better EPA additives and NO VI improvers (polymers) as in mineral based oils, which can breakdown with excessive heat and clog areas, and create that nastly sludge you find in an engine. They have extended drain intervals meaning you can forget about it for longer....typically double or triple that of mineral and semi oils. However, their exptreem protection capabilities can pretty much prevent metal to metal contact and limit the bedding in process which is vital to ensure peak engine efficiency.
However, this doesnt mean mineral or semi is bad, its perfectly fine for most road engines except for those exngineered on the extreem side. Most will run a semi of sorts, which is perfectly fine and does just as god a job as protecting as fully synth, but has a shorter drain interval, typically 3-5K for those who care....more if you dont.
Mineral oil has its benefits, particuarly when running in a fresh engine. The msit important part is the first few mins of running, and being a pansy will see your car not performing to its best for another 10-20K miles later. Its all about how the piston rings seal. Fully synth wont allow a fresh engine to seal efficiently, mineral will as is EPAs are not as effective and its film shear strenth not as good. However, expect to change oil every 3K max.
Hope this clears some stuff up, dont get caught up in nonsense and just buy the oil which best suits your needs....if you do alot of trackdays etc and run ultra high temps with no oil cooler, think of a fully synth 15w50 etc.
