Several things:
First of all, engine oils don't actually break down just from sitting, either inside the bottle, or in your engine. Several things occur to the oil while it sits there, doing nothing:
First off, depending on the quality of the oil (in this respect, poorer quality oils), some of the additives in the engine oil will settle to the bottom of the pan, or bottle. This takes a long time to occur. Its easy enough to shake a bottle before pouring it in, but if you can shake your car before starting it, you can do whatever else you want in life. I won't try to stop you.
Second, when the oil is in the pan sitting for a while, depending on the climate you live in, condensation might build up inside the engine. now technically, this shouldn't happen, as the engine is a fairly well sealed unit now a days, but people are convinced that it actually occurs. All you have to do is get the engine good and hot, and the condensation turns into water vapor in the engine, and gets sucked into the intake system to be happily burned in the combustion chambers. No harm, no foul.
Third, any combustion by products that are mixed in the engine oil will have a chemical reaction to the additives in the oil that are there to neutralize them. This then weakens the oil over time. Every time you start the engine and run it for a while, and then shut it back down, you add more by products, and you decrease the amount of protective additives. Eventually, you will have contaminated oil. Only solution is to replace the oil with fresh oil. This is the most common issue faced with cars that for sit long periods of no use.
And last, its not 5W-30 that is recommended in place of the 0W-40, it's 5W-40.
I'm about to make you blow your top with the next sentence, so be warned:
A proper search would have told you that.
Hehhehehe.
Aren't I a trip?
Anyway, next up is that Mobil 1 0W-40 oil actually isn't that good for sitting long term in an oil pan with combustion by products sitting in it. It doesn't have a good enough additive package anymore in order to neutralize them over the long term. You would be better off with another brand of oil, with a better additive package for that kind of lack of use.
BC.