I wasn't really underestimating migration, but I was keeping everything in mind. I knew the surname Diesel was German, but the Hispanic Diesel threw me off, seemingly pointing of another possible origin that I maybe missing. I've looked at another surname, Dietrich, a very common surname, and this surname is found throughout Europe, also in South America. The surname Diesel is a pet form of Dietrich, meaning the two surnames are related.
I have researched 100s of surnames, and I know not to make the same mistakes again. My point is, we shouldn't really narrow down the origins of a surname. For example, the surname Fish, a very common surname. Not all people with the surname Fish are of English heritage. Some are German, Spanish, Native American and others. If so, there is a possibility that their meaning & origins is not the same with the English Fish, meaning there is of another origin. Same with Diesel. There are the Filipino and Spanish Diesel, and if so, there is a POSSIBILITY that they aren't from the German theudo OR they did not originate from Germany. I understand about the migration of South America, but as you can see many of them are Spanish people, and there are many Native Southern Americans too, and If there are Diesels in South America, then there is a POSSIBILITY that there are Spanish or Native Americans that do bear this same surname, which I have found many Hispanic people with this same surname. I just PROPOSED the possibility, and many times I get it right. There is no reason to disregard possible origins based on speculations and hypothesis, because usually they are right. It just needs evidence to support that it is an origin, or it is not.