The engine manual probably said 30 weight for summertime use. It probably specified the API Service Category like, for example, category SJ or SL oil.

For an extended usage including cooler spring and fall use, I use Mobile One 10W-30 synthetic oil in my tiller. If you wanted to use the tiller in the winter, which is unlikely, Mobile One 5W-30 synthetic is what I would use. That's not an issue here in Maine, because our soil is frozen as solid as concrete in the winter.

Check the oil before each use and if it begins to get dark, change the oil while the engine is still warm from that use, to get a good drainout. If you had the engine manual, it would tell you how many hours of engine use between oil changes. Better to err on the side of changing oil too frequently.

If you don't know the oil capacity of the engine, make sure it is level when you add the oil. I actually use a bubble level for that, because just a little tilt can make a significant difference. I know my engine's oil capacity, but if it isn't level when I add oil, some oil can run out of the fill hole before I get enough oil in.

MM

For summer use, HD-30 from Castrol is what I use. I believe your engine holds 20 ounces. Change roughly every 20 hours of use, or yearly, whichever comes first. If there is a way to check the oil in the gearcase of the tiller, I would also do that. Make sure to check the oil level in the engine before every use. Also keep the air filter clean, and she should be good to go!