What is the difference between an SAE 30 vs SAE 30W oil? I'm curious because I could not find SAE 30W down at the local Autozone store. The operator's manual for my father-in-law's Toro lawn mower calls for SAE 30W detergent oil with API SG through SL. I picked up a bottle of Valvoline SAE 30 with API exceeding SM/SN. They only had SAE 30 oils (both detergent and non-detergent types, I bought the detergent type). So, my concerns are: is SAE 30 going to be OK? And should I have bought one with appropriate API service rating (SG through SL rather than SN/SM) and where would I buy it since I can only find SN/SM at local parts stores? I put about 4-5 ounces in the lawn mower to top it off to full, ran it for about an hour. Ran fine as far as I can tell. Then I noticed the oil I bought was SAE 30 and not SAE 30W. Double checked the manual... [censored]. Probably not a big deal. I'm guessing most oils are meant to be backwards compatible for most motors. Then again, I know jack about motor oils. Doing some research on it now, but thought I'd ask all you oil gurus out there while I'm at it. Also, I emailed Toro with my questions. Figured they may just give me a pat answer and repeat what it says in the manual, which is why I came to BITOG first, but they may shed some light on this for me. I will update when they respond. Any advice is much appreciated!