I don't want to argue much on this, as before I change to Wira, I drive a 94 Iswara, all the above mentioned oil I have used without adding addative. The result is using fully syn-oil have to top up another 2liters after 2 weeks, semi-syn have to top up 1liter after 1month and lowest grade no need to add. I complete 5000km around 3months or lesser. You can find out this with your mech on how true is my statements.
Another thing is if I not mistaken, the engine oil temp is more 50°C when the car is running.
http://www.twf8.ws/new/tech/tip/enginetemp.html
Just get the info from this website, the temperature is at 90-110°C.
If use the lowest grade for 3 years engine, you will feel that your car is abit heavy and response slower, if use syn oil you can feel that your car is like flying.
Frankly speaking I don't know what is the 15w50 or 20w50 stands for, maybe like you say I misunderstand it(I'm sorry for this) But I'm type of man that try and error, futher info you can check below website:
http://www.bpoil.co.za/products/engine/vanellus/c6global.htm
From the spec(10w40) you can see at ambient temp the visco is 106, when at
100°C the visco is 14.
You can also check Shell's website for the correct oil for the age of a car:
http://www.shell.com/home/PlainPage..._lubricants/helix_new/app_choose_ga_0312.html
You see, they only recommend you to use lowest grade for a 10 year car. Why they never ask you to use shell helix ultra?
http://www.blackstone-labs.com/oil_viscosity.html
I have read few times already on this, I cannot understand it... Haha.
From the above article also they not recommend we to use the addative. But the addative that I buy normally is around 50-100ml or lesser, brand NASA. maybe if the addative keep too long it will degrade, but my usage is fast, every change only takes 3months or lesser. I only come across to use the addative when I know that the engine oil lost 20% after 3 months. So after adding it, the lost is around 5% now, without performance drop, I have use 1 time the lowest grade and I feel my car abit heavy.