What oil to use in your equipment is one of those questions that is almost as bad to bring up as politics, okay maybe not that bad. But, what oil to use is one of those topics that once you ask you wish you could un-ask because everyone seems to have an opinion and often each opinion is different in some way. With that in mind I am not going to give my opinion on a brand. But what I would like to write about is the types of oils out there and what those funny little numbers mean.
As always the first place you should always start is with the manufactures recommendation. Keep in mind these companies put allot of money into research. But, what you can safely ignore is the companies trying to sell you their particular brand of oil.
The first thing you need to start with is what type of machine is the oil for? A riding lawn mower for example can use oil made for a motorcycle, but not the reverse. Machinery that uses a wet clutch (meaning the clutch plates are in the crankcase and covered in oil) need some additives that are generally not used in your regular automotive type oil. Next you need to make a determination about the conditions and temperature you will be operating your machine. Once you have that little bit of information you are ready to make an informed choice.
Those strange little numbers are important, and like many things in the mechanical world bigger is not always better. There are two general types of oil you will run into. Multi-viscosity and and single viscosity (also known as "straight" oil) Most engine manufactures call for a multi-viscosity oil. Viscosity is one of those fancy terms that simply means a liquids resistance to running. For example water has an extremely low viscosity, where as honey has a very high viscosity. Multi-viscosity oils are designed to have a low rating when cold and get higher as the engine heats up. This helps with cold starting and reduces engine wear. Straight oil does not change viscosity (in theory) which means that when the engine is cold the oil is fairly thick. Which on a lawn mower engine that does not have really tight tolerances it's actually a good thing, but not on a high performance motorcycle engine.
So here is a rule of thumb, if you are operating your machine in cold weather you want lower numbers, for example 10w-30, if you are in a hot climate you may want to run 20w-50. In my own bikes I use a 10w-40 oil year round. In my equipment I use 15w-40. Running 20w-50 oil when it's 20 degrees out will make your machine harder to start, and may even prevent oil getting into those really tight places until the engine warms up. So, if you are running a "heavy" oil in cold weather it is very important that you give your machine time to warm-up before you take off and put a load on it.
Whatever oil you decide on, you should try and stay with the same brand and weight as much as possible and unless your engine is very very very clean inside already avoid using additives that "clean". If there is any built up crud in your engine a cleaning additive can break it loose and that debris can clog an oil passage and lead to catastrophic failure.
Finally, remember to change the oil and filter (if there is one) regularly. Even the best oils will begin to break down over time and become contaminated from the engine with use. The best way to ruin an engine is to ignore the oil.