It depends upon what the manufacturer recommends and what the duty cycle is like, not to mention ambient temperatures. I've used everything from 5w-30 conventional in resource conserving to 0w-40 A3/B3 A3/B4 to HDEOs to actual monogrades.
If it's going to be hot and used a lot (i.e. a city owned or contractor owned air cooled lawnmower), it would probably be best to go with the monograde if recommended by the manual, with an SAE 30 usually being sufficient. For stuff on the farm, that got a fair bit of use, but also might be put into use in cooler times like spring and fall, 10w-30 was used year round. In such a case, I'd use an HDEO 10w-30, since ILSAC oils weren't the norm back then.
For me, who uses such equipment comparatively rarely, being just a homeowner, for rather short periods of time (small yard), and with few climate worries (snowblower is in the heated garage), I use whatever straggler quarts I have laying around. I think I mentioned in another thread here that my lawnmower current has SAE 30 in it, since my small engine guy had to play with the carb. Before that, it had MaxLife 5w-30, non-ILSAC version. My snowblower has RP 0w-40, thanks to a quart laying around. If it were outside all the time in our weather, I'd be wanting a 0w-XX in it, like that RP 0w-40 or Mobil Delvac Elite 222 0w-30.
Basically, I'd say if you're working the stuff fairly hard and in a hot climate, I'd grudgingly recommend an SAE 30, maybe an SAE 40 if preferred by the manual. For me, though, the fuel systems are going to be rebuilt three or four more times, by which time I'll be sick of the equipment, and junk them, with perfectly good engines. For stuff used year round and quite a bit, an HDEO would be a good option, giving appropriate HTHS and a suitable cold cranking capacity, if the correct grade is chosen.
tig1's recommendation above is another very reasonable idea for equipment that gets a fair bit of use. Hunting for an SAE 30 isn't as convenient as happening to have a bottle of M1 10w-30 (or equivalent), which is plenty capable, on the shelf. A lot of manuals I've read treat 10w-30 and SAE 30 about the same, with a little more "worry" about 5w-30 when it comes to consumption.