5w is thinner that 10w this is why its best to user 10w on dizsnizles, or higher milage, 5w is good i use 0w Castrol RS its a super fine oil that gets in everywhere but does at times burn abit as it passes through the rings a little easyer, 10w is slightly thicker as mentiond with the looser engines and thinner oils it can burn off through the rings the other part of the 5w- is the hot spot or optimum temp if you have a hot running engine you want a higher number but most cars are aroung the 40 mark hence 10w-40, if you run your golf with the 85 temp sensor that keeps the temp down id id advise you to look in to maybe using somthing like a 5w-35 maybe im blowing out my ares but this is going from memory working for vauxhall, all in all magnatex is a great oil what ever one suits your engine better, i use RS as its even better and what was used in her before i bought it, if you can help it down mix oils... magnatex will work after about 3 changes to get rid of the old other type
shut up GSN for got sake.....
G