i read with great interest a thread the other day concerning, among other things, the wisdom of using 15-40 oil in mower engines. I understand the principle of multiple-viscosity oils, but not the chemistry. However, there is one thing I seemed to glean from all this is that the less the range in viscosity the better. In other words, many felt 15-40 oil would be superior to 10-40 because it has to have less additives to accomplish the viscosity change. At least I THINK that is what you guys were saying. Now, for my question. I've been using full synthetic oil in my mower ever since I bought it. Since 40 viscosity oil is recommended at ambient temperatures of 90 degrees or so and above, I've used 10-40 synthetic, in a couple of brands. Well, the last two times I needed to change the oil, I haven't been able to find any locally. Instead, all the major brands seem to be switching to 5-40 instead, so I bought some Rotella full synthetic in 5-40. I change my oil every 50 hours, and the other day, with 40 hours or so on the last change, when checking the oil, I found it to be a bit low. Not quite to the add mark, but this is the first time I've noticed any usage between changes (engine has approximately 400 hours on it). It has, however, been hotter than last year, and I operated it in triple digit heat a couple of days, so that may well be a factor. In the opinon of you guys who really know and understand all this lubrication stuff, is 5-40 OK to use, or should I try and find a source for 10, or even 15-40? I would really like to stay with synthetic oil. They all seem to manufacture 15-50, but since Kawasaki and Gravely don't recommend 50 viscosity oil, I am reluctant to use it, thinking it might be too thick. Thanks in advance for your help. I'm confused.