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Really you are only interested in the second number in the oil weight during the summer. 5W40 means the oil acts like 5 weight oil when cold (20C I believe) and 40 weight oil when its hot (100C). The only reason these multigrade oils exist is to aid starting the engine when its cold and to reduce wear as the thinner oil starts circulating a bit quicker.
If you add xW30 you are thinning your oil at operating temperatures, probably fine if your engine is in good shape, maybe not a good idea if its old and getting more worn out.
In the summer in poland conventional oil 15W40 is fine, during winter is really when you need 5W40 which is usually synthetic. Also make sure you are adding diesel oil, not gas engine oil.
Ian
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