Here is my basic question, what is up with all the different oil types? and how do I know what is right for my truck?

The reason I ask that question is because I was reading through my manual for my truck for no reason really when I stumbled upon this.


The Oil that I've always been putting in my truck has been the Shell Motor Oil, here is a picture of the back.


I've been using this stuff for the last 25k miles in my truck and have never had any problems(changing every 3-4k miles of course). Although I noticed after I read my manual that there is no SG anywhere on the back of this oil bottle. This is what Autozone sold me the first time i went to get oil, so i just kept getting this kind. After reading my manual I went back to Autozone and looked at every single bottle of oil they had, and NONE of them had API SERVICE SG. Except for this one below, and it was like the cheapest bottle they had too. Its not actually in the symbol but above it. Take a look...


I bought the stuff in the last picture the last time I changed my oil. I haven't noticed any difference in how my truck is running, it seems to be the same. But is there anyone that is an Oil expert and can tell me why there doesnt seem to be SG oil that exists anymore? And will either of these that don't have SG on them be ok to continue putting in my truck?

I just don't want to harm my engine

 

SG is an old API rating, it was superceded long ago. SM is fine for your truck. For more info follow this link.

 

Thank you and a very speedy reply nonetheless!