I think you made a fine choice, both in viscosity and brand.
My 38 hp Ford 2810 specs 10W30 up to 90F. Check your manual.
IMO, a lot depends on how you use the tractor. I don't know the Shibaru engine (mine is a Dagenham tractor) but, generally speaking, if an engine is not worked super hard continuously for days on end, and the oil temp stays below 212F, you are very good with a 10W30 HDEO in a small tractor engine. If it's a short hopper, used relatively lightly or for short periods at a time when the oil doesn't get fully warmed up (it takes 20 minutes of work to get the 8 quarts of oil up to full temp on my little tractor), the lighter oil is even better.
I ran 15W40 in the 2810 for a while but one very cold winter morning, I had a no start. Couldn't spin over fast enough to start. Switched it over to 10W30 and it's started on it's own at subzero temps. I have since added a block heater and also have electricity in the barn now so I can plug it in.
I went through a learning process after that no start and ultimately, it changed many of my attitudes. At first being a bit leery of straying from the "everybody does it" 15W40, I started some research. With more investigation, I discovered methods to determine the efficacy of a viscosity change. I started by measuring oil pressure and oil temps to determine if 30 grade would work in warmer weather. There was about a 4 psi drop in OP between 15W40 and 10W30 at operating oil temp (around 185F). No matter what I've done with that little tractor (it's my utility here on the farm), including chopping wheat stubble on hot 90F days, the oil temp doesn't exceed the 212F viscosity temp, so the oil is staying well into 30 grade and at 185, it's into 40 grade. A 40 grade in those temps would be in the 50 range and that's needlessly thick.
It could go the other way, where the oil exceed 212F and begin to drop lower in viscosity. If you were going to "play" with viscosity, you'd need to know these things.
In the end, after looking at two tractors, a diesel pickup and a gas pickup in this way, and comparing what I learned to the manual's recommendations, in those cases, the manual was right on. The OEM figured all this stuff out for me. I can't speak to other vehicles, but in my narrow sampling, I think the visc charts were spot on.
