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Friends with BMWs use 15W50... would that cause any problems with the IS? I know the manual says 10W30...
 

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The manual (oil cap has it too) recommends 5W30, not 10W30.
10W30 is recommended for hot areas.
 

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The smaller the difference between the two numbers, the better. The numbers are a function of the viscosity at hot and cold temperatures. The lower the first number, the oil the better the oil in cold temperatures. The higher the second number, the better the oil will perform at extreme temperatures (engine temp...not air temp, although engines are under more stress at high air temperature). To have a oil perform well at low temps, the composition is different than at high temps so you end up having a compromise and the oil is less stable with large differences. 10w30 is marginally better than 5w30 in stability, but 5w30 will perform better in subzero temps. In houston, the temp rarely gets below 32 F, let alone 0, so the 5w30 doesn't really provide me any benefit.
 
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