the hotter the oil gets the more it thins out...which made me think that it could have negative results.
actually you're right, there is something negative: as a fluid film gets thinner it may tear, i don't know if i picked the right word for it, but let's say at certain temperature the film will leave you and there'll be direct fricition.but:
thinner oils are designed to operate within almost every temperature range an engine may be working in, so don't worry about using thin oil, a normal engine won't get into temperature heights where the oil film may tear. nonetheless there are graphics for operation ranges of engine oils, generally, by type. i'm too lazy to look this up, but it shouldn't be too hard to find.
you may divide oils into two types (operational point of view): mono grade and multi grade. so where's that from? mono means they are made for a small temperature range (example SAE 40), works in mostly tropical climate, no major temperature changes throughout the year. multi grade means they have a wider temperature range (example SAE 10W 40), "W" means "winter" (i may be wrong with that one). so they are sold in areas where temperature changes throughout the year, most US for example. the smaller the number gets, the thinner gets the oil, a SAE 30 is thinner than a SAE 40, and its temperature range is also lower, for example: SAE 30 from 0°C to 130°C, SAE 40 from 5°C to 140°C. Thats only an example, nothing to do with real numbers, just to explain a bit.
you also may imagine a mono grade for winter, high up north for example, they might need a SAE 10W (never seen though).