I have a question, I have an e46 316i 2004 (1.8L 115hp). The manual advises to use 5W30 LL04 oil. The previous owner did the maintenance 4000km ago and he used 10W-40 which is not meant for this engine because it’s half-synthetic and not bmw approved. My brother says that once a thicker oil has been used I can’t go back to thinner oil, is this true ? Can I change my filter, drain the old oil and put 5W-30 back in or not ? An alternative is 5W-40 which is full synthetic and has BMW Longlife01 spec.
No.
Yes.
10 character s
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As was already mentioned, the OP’s brother is wrong.
I would recommend that the OP change the oil & filter immediately, using the correct specification oil.
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And after you change it, keep an eye on the engine looking for signs of a leak. Sometimes people will run a heavier oil to get rid of an annoying oil seep instead of replacing whatever seal is leaking.
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It appears that you just recently purchased this vehicle. It is 14 years old and has probably had all kinds of oil in during that time. Just put the correct oil in at the next change and look in the owners manual for your info or a BMW mechanic rather than your brother.
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Your brother is completely wrong. The engine has no idea that it had the wrong oil in it.
Do an oil change using the proper oil and you are good to go.
Now be sure to take what your brother says with a grain of salt!!!
Yosemite
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Thanks everyone! I’ll be keeping an eye on my oil level and make sure to check for leaks too 
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