I don't mean to be rude here but you are quite mistaken about the viscosity the engine is designed for, as it is designed to operate a HUGE range of viscosities over a wide temperature range. See, 5w60 is no thicker at startup than a 5w30 during the northern winter. In fact, even at 25 celcius, the difference is minimal. That number after the "w" is measured at 100 celcius. It means that at 100C, the oil has still gotten thinner (as oils always will), but it has only thinned out as much as a monograde 50 weight would. But when the oil is cold, it doesn't get any thicker at zero degrees than a 5 weight would. So interestingly, a 10w50 is still thinner at operating temperature than a 5w30 is at startup, therefore, the oiling system will not have any issues moving it. In fact, the bigger the difference between cold and hot numbers, the more stable the oil's viscosity is over a temperature range. For example, a 5w50 maintains it's kinematic viscosity from 0 degrees to 100 degrees much more closely than a 5w30. This is referred to as Viscosity Index, and the higher the better in most cases. The better the viscosity index, the more expensive the oil is, generally speaking.
VI is best explained in this graph:
EDIT: note the overlap at 0C and 100C. This is what makes it a 5w40 oil
Here's some numbers to show what i'm talking about. Note: Kinematic Viscosity is measured in Centrestokes or cSt
AT 100 CELCIUS:
SAE 30 = 9.3 - 12.5 cSt
SAE 40 = 12.5 - 16.3 cSt
SAE 50 = 16.3 - 21.9 cSt
SAE 60 = 21.9 - 26.1 cSt
AT 0 Celcius: (estimations from plotted data, as cSt in not normally reported at 0C)
0w = ~450 cSt
5w = ~550 cSt
10w= ~750 cSt
15w= ~1300 cSt
these are some very high numbers (but the oiling system copes with them on a cold day), we don't normally use the cSt rating at this temperature range
These are MULTIgrade oils we deal with, so the second number is irrelevant at 0 celcius, so that's where the 0w, 5w etc comes from:
BUT, AT 0 CELCIUS: we actually measure Cold Cranking Viscosity, not cSt:
0w = 6 200 @ -35°C
5w = 6 600 @ -30°C
10w= 7 000 @ -25°C
15w = 7 000 @ -20°C
The oil recommendations for this engine actually include viscosities from 5w30, right up to 15w50. You would select them based on what you do with the engine (hard driving: use a heavier hot weight)
We can now see that oil viscosity does not change linearly, it changes at a curved rate dependant on the two numbers that define it's "start" and "end" points in cSt:
Best shown here: (warm-up shown only)
So as you can see, the change is viscosity during initial warmup is effected very little by the second number, mainly the the first number is what's important, and even the heaviest 20w50 is still thinner when warm than the 5w was when cold. So the oil pump will have no issues moving it around.
Now, let's not forget sheering effects. You might find a 5w50 mineral oil, sure, but don't touch it. A mineral oil required Viscosity Improvers to achieve this excellent VI. They stop working over time, ie. they "sheer" (quite literally, the molecules get sheers into smaller molecules under heat and pressure in the engine), leaving you with something more like a 5w20 as the oil gets towards it's drain interval...lower viscosity when hot = thinner oil film in things like bearings and piston rings = more wear (and at a hot rating of 20, wear would be relatively fast).
Fully Synthetic oils, however, offer the advantage of being able to go without Viscosity Improvers, they have the correct sized paraffins, esters and olefins to achieve the viscosity index required to make a 5w50 oil. They do not sheer nearly as easily, if at all, during a normal drain interval. For this reason, you can also go an extended drain interval with many synthetics. As long as they are Group IV or V oils, and not the much cheaper group III synthetics. You get what you pay for basically. Remember to look for an API: SN/CF and ACEA: A3/B4 if you want to be sure you are getting the "good" oil.
So in conclusion, if you engine is not brand new, you can benefit from using an oil with a higher "hot" rating, as it will hold a thicker film through the larger bearing clearances etc. You can not hurt your engine by using 40 or 50 "hot" weight oils such as 10w50. But don't go higher than this unless you have a lot of miles, because you may reduce the overall oil supply to certain parts of the engine, which may cause sludge formation or loss of lubrication.
Technically, the best viscosity to use would be 5w40 or 5w50 or even 0w40, 0w50 (but you would be wasting money at this stage). Because on a cold winter morning, it is very easy to warm up to working viscosity, but it doesn't get too thin at operating temperature.
Another thing to bear in mind is that when this engine was built, oils of these grades were basically unavailable, and so were not included in the recommendations. But these grades are BETTER than what used to be available. There is gain to be had for engines that are worked hard, or have some wear already.
For engines that don't get driven hard, do mostly long drives and not a lot of city driving, get changed at 5000km intervals, then sure 5w30 will absolutely be sufficient
Lubrication Consulting and Lubrication Training
Oil Viscosity - Engineers Edge
Motor Oil Viscosity Grades Explained in Layman's Terms
and most importantly: my experience as a Process Chemist at Australasian Lubricants Manufacturing Company (ALMC), where Caltex, Castrol, BP gear, engine, hydraulic and agricultural oils and greases are made.
Sorry if i ranted for too long. I didn't know how to explain this any shorter :/