No problem, I may help you.
Forget the shop, which is at Kadıköy.
All you need is a pencil, some paper and physical science.
For example you need "100 cc 7 W" oil and you have 5 W and 15 W. You should mix them to get some 7 W oil. Start with calculating;
(a x 5W) + ([100 - a] x 15W) = 100 x 7W
5a + 1500 - 15a = 700
10a = 800
a = 80 cc
Then mix them, you should use 80 cc 5 W and 100-80 = 20 cc 10 W.
I wish i could help you.
Anlayana...