There so many old wives tales & misconceptions in the world on motor oil that you really should not trust anyones advise. After all... It's not their engine that may seize & I doubt they will be there to help you if your engine locks up. You need to educate yourself on what your machines needs are & what form of lubrication best suits those needs. Also you need to understand that a quart of Valvoline (or whatever brand you prefer) 40 weight oil that you buy today, is not the same oil that would've poured out of the same container 20 years ago. In many ways it is better, but in some ways it is worse. The main problem for our engines is that much to all of the zinc has been removed from over the counter oils intended to be used for street vehicles. I believe the reason being that the zinc is not only a pollutant but it fouls the O2 sensors & cat cans in modern cars. Zinc is a very important additive for engines with flat tappet cam shafts. Most modern vehicles have roller cams these days, so the zinc is no longer needed as much as it used to be.
The contact point between the cam lobe & the lifter is an extremely high wear area. This is also the main component that defines your engines performance characteristics. High lift cams usually require stronger valve springs to control the valves action & insure that the lifter doesn't bounce off the lobe. Of course this increases the load & wear at the lifter & lobe. So far, Zinc has been the only reliable additive to provide the proper lubricating properties required in this area.
We also need to keep in mind that our engines depend heavily upon the oil to provide cooling. These engines are not 'Air Cooled'.
ut: They are Air & Oil Cooled. The desert boys, grow up learning a few things about oil that most folks don't know. When it's hot... Thicker is better. My dirt bikes always received Torco 50w, year round. As someone has already mentioned, the multi-grades are not well suited to our engines. I have seen this in my lawn-mower. I once used some quality 10-40w due to not having a straight 40w at the time. Oil consumption increased dramatically. Also, we usually don't have any oil filtering system in place. A magnetic drain-plug (If possible) would certainly help. Bottom line is that we need to dump that oil more often. Especially on a new engine that is wearing in & creating thousands of sharp metallic particles to be splashed around & scratch the hell out of the inside of your engine. I've always tried to use the highest quality oil I could find for all my engines. Spending a few $$'s more on oil is just plain cheap insurance.
Back to the zinc... Oils that are being sold for Racing use & Diesel Engines still have adequate zinc levels to the best of my knowledge. There are also zinc additives on the market that allow you to use what ever oil you'd like & then add the zinc. Here is one that I have used in the past:
ZDDPlus™ - ZDDP Additive for Classic Cars - Agricultural Equipment & More I recently saw that AMSOIL is producing a synthetic designed for small engines, ATV's, etc, that has zinc added. I bought a quart & will be trying that out soon.
There is plenty to read on the I'net about the quality of various oils, zinc, filters, etc. Here is a link that will get you started:
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