You really DON'T need a 50 weight oil.....
A couple of reasons. Your motor is designed for a 30 weight. The oil passages, bearing clearances and such are designed for 30 weight.
Going to a 50 weight, the oil is heavier and will not flow as well in your engine. You also could cause more wear until the bearings wear in for that weight of oil, then you cannot go back to a 30 weight. You also may also see a loss in MPG. Oil is the primary method to cool the lower end of your engine. If you don't have enough flow, heat will build up.
On engines designed for thinner oils thicker oils cause more friction and therefore more heat and they waste power and affect fuel consumption so it's always best to use the thinnest oil that you can get away with and still maintain oil pressure. Use what BMW recommends.
Any good synthetic will be just fine. I use (for example) Amsoil 0W30 in the Roadster (2.5i) and change it every 7500 miles. The car has 66K miles on it and does not burn any oil.
An excerpt from my webpage on oils:
THE THICKER THE BETTER
Not so fast, there. This isn't always true - even when using a petroleum oil. Although it is true that
heavier viscosity oils (which are generally thought of as being thicker) will hold up better under heavy
loads and high temperatures, this doesn't necessarily make them a better choice for all applications.
NOTE: When I refer to higher or heavier viscosity oils, I'm referring to the "second" number of a multiviscosity
specification. In other words, a 10w40 would be heavier than a 10w30 because 40 is higher
than 30. I'm not making any reference to the "w" rating because this doesn't come into play unless
you're referencing cold temperature performance.
You see, on many newer vehicles only 5w30 or 10w30 motor oils are recommended by the
manufacturer. If you choose to use a higher viscosity oil than what is recommended, at the very least
you are likely to reduce performance of the engine. Fuel economy will likely go down. Engine
performance will likely drop.
In the winter months I would highly recommend that you not use a heavier grade oil than what is
recommended by the manufacturer. In cold start conditions you could very well be causing more engine
wear than when using a lighter viscosity oil. In the summer months, going to a heavier grade is less of
an issue, but there are still some things to be aware of.
Moving one grade up from the recommended viscosity is not likely to cause any problems (say from a
10w30 to a 10w40 oil). The differences in pumping and flow resitance will be slight. Although, as was
mentioned, efficiency of the engine will decrease, the oil will likely still flow adequately through the
engine to maintain proper protection. However, it will not likely protect any better than the lighter weight
oil recommended by the manufacturer.
Moving two grades up from the recommended viscosity (say 10w30 to 20w50) is a little more extreme
and could cause long term engine damage if not short term. Here's the thing. Although the oil will still
probably flow ok through the engine, it is a heavier visocosity oil. As such it will be more difficult to
pump the oil through the engine. More friction will be present than with a lighter viscosity oil.
More friction will be present than with a lighter viscosity oil. More friction means more heat. In other
words, by going to a heavier weight oil in the summer months, you may actually be causing more heat
build-up within the engine. You'll still be providing adequate protection from metal to metal contact in
the engine by going with a high viscosity, but the higher viscosity will raise engine temperatures.
Over the short run, this is no big deal. However, over the long term, when engine components are
chronically run at higher temperatures, they WILL wear out more quickly. As such, if you intend on
keeping the vehicle for awhile, keep this in mind if you're considering using a heavier weight oil than the
manufacturer recommends.
The key is to generally stay away from viscosity grades that are not mentioned in your owner's manual.
Sometimes vehicle manufacturers will make reference to the possibility of using a 10w40, 15w40 or
20w50 motor oil, even though they might recommend a lighter weight oil for most situations.
If this allowance is made, you'll be ok using a heavier grade of oil. You'll probably see a drop in mpg
using the heavier oils, but you will not likely cause any long term engine problems. I would recommend,
however, to stick with the lower weight recommendations if using a synthetic oil, even if you're running
the engine in higher temp climates. I think you know why by now.
If the manufacturer does not make allowance for heavier weight oils, it would be my recommendation
that you DO NOT use a heavier weight oil in your engine. Under most circumstances, stick with a
viscosity grade that is recommended by the manufacturer. And, if you do choose to use a heavier
weight oil, at least make sure that you only move up one grade. Never move up two grades.
