I bought a poulan pro 6.25 series with no manual please tell me the correct engine oil to use for a PR625y22RP engine please

I'm guessing you have a basic Briggs and Stratton engine, which is the standard on these machines (or a Tecumseh). The standard oil used in almost every mower on the planet is straight 30 weight oil (not multi-viscosity, such as 10W30). Just 30 weight. There should be a label on the side of the engine and on that label, it will actually be listed, but I'm going with 30 weight. For longer engine life, use synthetic oil. It will need less changing. Here is a photo off one of my Tecumseh's. You'll see it clearly states SAE 30 right there on the sticker. Hope this helps.