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if the oil body is different the sound will change...did you use oil of the same viscosity?
5W-30 is the preferred oil viscosity
not much. it would change the viscosity of the oil. the diesel oil is much thicker than regular engine oil.
decreased temperature
The 5 and 30 refer to the relative viscosity of the oil when it is cold (5W) and when it is hot (30W). A 5W-30 oil behaves like a 5W thin viscosity oil when the engine is cold, and it behaves like a 30W thicker viscosity oil when the engine is hot. A very thin viscosity is undesirable in a hot engine. In very high temperature situations, an even higher viscosity range, up to 50W, might be called for. Always go by the engine manufacturers recommendations.
Yes. If the temperature changes enough, the viscosity of oil will change, too. Oil is designed to resist changes in viscosity with temperature changes, at least to a degree. But if the change (either hotter or colder) is extreme, then the viscocity will most certainly change. And, as you'd expect, as it gets hotter, the viscosity of oil decreases (it get thinner), and as it gets colder, the viscosity of oil increases (it gets thicker).
The wrong viscosity of oil in your engine.
5W30
It depends on the condition of the engine. If the engine is relatively new, you can use a "Synthetic" oil that will keep the sludge from building up. If the engine has more than 50K mi, don't change from the brand you've been using. If the oil pressure is low at an idle, change to an oil that's a little "thicker", meaning that it will have higher viscosity numbers. In general, you want to use the lowest viscosity numbers possible that will keep the oil pressure up.
30w
The viscosity of the oil may not be heavy enough to protect the engine.
If the light was on before you changed the oil and you now have the correct amount of oil, and correct viscosity, then it can be that your oil pump is defective. not putting out enough pressure. if you have an oil gauge, if that shows oil pressure is OK, then your light can be shorted out.