This is a sticky subject and there are lots of opinions and theories on it which can all be in part true or not. It just depends on how you look at it and what best suits YOUR needs. Here's what I was told by an old friend of mine from Redwood oil, who was an refining engineer, so he should know what he's talking about. There are basically 3 kinds of oils, ash based, paraffin based and synthetic. Ash based oils are what I prefer. They are commonly Valvoline, Kendall, Castrol and a few other's. Paraffin based oils and "wax" based and you have probably seen old, neglected engines where you take-off a valve cover and it is literally caked with crud inside with this light weight, dry substance stuck to everything and clogging everything up. That my friend is paraffin wax from the oil. You'll hear old time engine builder's call engines like that, "Pennzoil engines". Don't get me wrong, there's nothing wrong with paraffin based oils (or Pennzoil) as far as protection goes, it is only when you leave oil in there too long that it starts coating the insides of everything with wax, and just like when making candles the old fashioned way, by dipping a piece of string into hot wax over and over again, it builds-up on the string until you have a candle. This is exactly what goes-on inside your engine if you don't keep the paraffin based oils fresh and clean.
Oil weights are another story. We used to mainly use straight weights, such as 30 wt. for standard engines and 40wt. and 50 wt. for racing engines. This was due to whatever the inside clearances were and temperatures the engines were going to be run at. A racing engine is usually set-up loosed (wider clearances) than a basic street engine and most racing engines are run in warm, Summer weather, which would require a thicker oil to take-up the spaces and keep the engine protected. These days, with multi viscosity oils, you get better protection BUT there is a real misconception on this. 10W-40 oil is 10 weight oil. It is very thin. 10W-40 means that the oil is a 10 weight based oil but when it's hot, it has the "viscosity" of 40 weight oil. It does NOT turn into 40 weight oil like so many people think multi viscosity oils do. They are called "multi viscosity" for a reason, not "multi weight" oil. It isn't meat gravy! It doesn't get thicker when it heats up. Don't believe me? That's an easy one to prove. Check your dip stick when your engine is cold. The oil stays on the dip stick as normal. Now check it when the engine is hot, the oil will run off that dip stick like it was thinner than water! Like I said, it ain't gravy! 20w-50 is the same thing, 20 weight oil when it's cold and the protection or "viscosity" of 50 weight oil when it gets hot. This is because the molecules in the oil are kind of like springs. When they heat-up, they expand and when they are cold, they contract, in simple terms. This expansion and contraction does not thicken the oil when it gets hot. It still thins-out like any oil.
Now, what is a good weight to run in your engine? Most modern car engines have very tight clearances and need thinner oils so the oil can get where it needs to go to do it's job as fast as possible. Thick oil has a hard time getting in tight places or getting where it needs to go until it warms-up and thins out. That's why we always warm-up our performance engines before we started driving or putting any loads on the engine. This made sure the oil was warmed-up, thinned-down and has had time to get where it needs to go to start protecting the internals of the engine. Thinner oils have less drag on the oil pump and drive gears as well, which equates to less parasitic power loss. But again, too thin of oil can also mean les protection under high loads and high heat so there is no one perfect answer. The problem with thin oils is that most oil and advertising is geared towards the East coast where it is very cold in the Winter and thinner oil is more suited. The West coast almost never gets that cold, so the oils we use out here need to be thicker. I would never recommend 0w-30 or 5W-30 oil in ANY performance engine let alone any normal street cars in warm weather. That stuff is like water and can't take the kind of heat and loads of a performance engine. This can go on and on, so I'll try to keep it brief. If you look in a Valvoline catalog under oil recommendations, they recommend 30Wt. in trucks that drive in weather over 80 degrees. Well, that is pretty normal weather for the West coast.
The nitty gritty of oils. Mineral oil is refined and the refining process is pretty good but not perfect. here's how my friend from Redwood oil explained it to me so it was easy to understand. Crude oil has everything in it, from gasoline to diesel to all of the weights of oil and each part is processed out of the whole crude oil. Once a weight of oil has been refined. What they are doing to get the different weights is separating the different sizes of molecules that make-up the thickness of the oil. Oil molecules are like little ball bearings that let metal to metal parts glide on them between a certain clearance. Thin oil has small ball bearing like molecules and thicker oils have larger molecules. Picture a bag of marbles, where you have 100 individual marbles. Now instead of having all of the marbles the exact same size, imagine about 80% of the marbles being 1" in diameter and 10% being 3/4" in size and the remaining 10% being 1 1/8" in diameter. Now lay out all of the marbles on a flat surface and lay a piece of wood over the top of it. What is the board going to be riding on? The small percentage of the largest 1 1/8" marbles that are scattered out under the board. Well, that won't make the board very stable and it won't be riding on the majority of the 1" marbles like it is supposed to be doing until the biggest marbles have been made smaller in size so they ALL can carry the board smoothly and glide it across the surface of the ground. That is very much like refined oil. Refined oil has a high percentage of the correct size molecules, BUT, there is also a percentage of carry-over sizes that are smaller or larger in size as well. They can't refine it perfectly. Your engine bearings are like that piece of plywood, not riding smoothly on ALL of the marbles, only in the largest one's which makes the load surfaces uneven and smaller. Now imagine the same piece of plywood with the same 100 marbles, only this time, they are ALL exactly 1" in size, thus dispersing the load over the entire surface of the plywood and floor. That would be the ideal situation and this is very much like synthetic oil molecules, where the molecules are not refined and sorted-out by size but rather are MADE synthetically to a very specific size which carries more of a load and disperses the load over a greater surface area. That is why synthetic oils work so much better than refined oils. The only problem I have ever found with synthetic oils is if you don't have an oil leak with regular oil, you probably will once you go to synthetic. That stuff just has a way of finding its way out of your engine.
So, what do we recommend as far as oil goes? We like using 20W-50 in our engines. Be it synthetic or ash based. We rarely use paraffin based oils and rarely use anything thinner than that. After all, it is only 20weight base oil and that is plenty thin for our climate and with the 50 weight viscosity once it warms-up, it gives us the protection we need for the loads the engine is going to be putting on the internal parts.