There are no such things as universal engine oil. 20W-40 or 20W-50 is not good for all engines. There is a huge difference between 20W-40 and 5W-30. Let’s look at the W side. If 20W weight oil is used in a 5W certified engine, two things happen every time you start the engine from cold. First, the cranking speed is slower and takes a longer time to start because the moving parts are dealing with heavier oil that imposes more friction. Secondly, the oil does not flow as quickly as required to effectively lubricate moving parts so some parts are not properly lubricated. Metal to metal wears increases. Again, the oil does not remove heat adequately from components that heat up first like the piston and exhaust valve. Wear is increased and other damages are more likely to occur. The dark color of used oil, and its thicker texture are largely caused by the fine grindings of the different rotating parts. Is this why engines don’t last long in Nigeria? So, stick to what your engine manufacturer recommends. I am not here saying you should immediately switch back to what it should. Not so fast! If you have been using a heavier oil than recommended, some engine parts may already be worn. Going back to a lighter oil may increase oil burn. If your crankshaft and its bearings are worn too, you need thicker oil to build enough pressure to lubricate it to extend its dying day. If you have used heavier oil for just a few times and everything still looks ok, you may try what was recommended and keep an eye on it