If you are just talking about lubricating the engine bearings, go ahead with whatever oil is of the right viscosity. If you are talking about oil to mix with the petrol for a 2-stroke engine, then use real 2-stroke oil not synthetic.
You can get the correct oil from Homebase etc and garden centres so best to use it. Motor mower engines need a single viscosity and not multigrade.
5W is too thin and will probably be burnt and make it smokey.
wouldnt lose too much sleep over it, i have a mower with a honda engine thats over 15 years old ( cost about £120.00), never given it an oil change yet ,just topped it up with whatever is lying around, still starts & runs first time at the beginning of the mowing season...
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