I'm going to throw my .2 cents in here, I'm no expert so please disagree with me if you get that gut feeling. This is all from research and experiments I've done in high school and a little in college.
First of all oil viscosity is different for all oils for the most part. Like shoes for example, a size 10 Nike isn't the same as a size 10 Puma. Likewise, 5W-XX from Mobile one isn't the same as a 5W-XX from Redline. I just wanted to touch on that with you guys really quick. Here is a chart for quick reference.
Alrighty now, so in case some of you didn't know we have 5 base groups of oils, groups I - V is how they go. Now today's oils in your local auto parts stores pretty much carry group III hydrocracked oils, aka out of mother earth. Group 1 I won't even mention but groups II's are those single weight oils you see on the shelves, nothing special there, it's break in oil if you want to use it I guess. Group III base stocks are the same as II's except we have many more viscosity's....SWEET RIGHT, we love choosing 5's and 10's and for you crazy people the 20's!!!! LOL okay so then we move on to the good stuff.
Group IV base stocks are polyalphaolefins (PAO). These were the 1st synthetic oils produced, example would be German Castrol European Formula, bitchin oil for the price, I put 12k miles on it no prob. Group V base stocks are for the most part poly esters (POE), quick example of that is Redline, superb oil but ehh $10 bucks a quart, not in my street machine. Here in the United States a group III oil is okay to be called a synthetic, not in other countries like Europe. Yep, that super cool "synthetic" penzoil oil you've been using isn't really synthetic, sorry. Doesn't mean it's a bad oil, not even close. Today's oils regardless of what you choose will run perfectly well in your motors assuming not big huge mods have been done to the bottom end. The reason i bring that up is because of the prices they charge for oil that is not a true synthetic…if I’m going to use a synthetic oil, I want to get what I’m paying for.
Just so you know I know more about synthetics than I do mineral oils for the fact that I own a turbo car. They resist higher temperatures, like in the turbo center section, before they begin to break down.
In case your wondering I run 0W-30 German Castrol in my Supra, I usually don't discuss why as most people freak when I tell them that to the point where they have to make ignorant jokes like "why do you pour water in your motor," but I will say I prefer flow to pressure. The quicker the oil lubricates my motor on cold start up the happier I am.
