I work for an Oil distributer so maybe able to help, the 10w/40 relates to the "vicosity" of the oil or the thickness if you like. The two numbers are clasifications for different temps they use to measure the flow of the oil.
In short 15w/40 is thicker than a 5w/40. be aware though most standard 15w/40 you pick up from a forecourt or such places will be mineral based, they are mainly desgined for old cars and I do not know of a modern car I have dealt with that uses this oil. Most modern cars will use a Synthetic based oil be it part or fully, Alfa use the 5w/40 fully.
Never use the wrong oil, if you have the 15w/40 in it now remove it ASAP, Short term fix will casue a host of long term problems. It will leave a thick residue over the interior components and ultimately cause huge wear on the engine, It will be even worse in winter months as the oil thickens further, likely to cause overheating or significant wear on the engine as it heats up without proper lubrication.
If I can be of anymore help, just ask,