Re: oil viscosity question....

Posted by Fish'n Jim On 2016/11/13 21:10:37

MIL spec eliminates alot of things, as well. You'd have to look up the particular one. They can be very specific to one product. But you're correct, if the mil spec matches they are interchangeable. eg, pure silicone oil made the same way by different manufacturers.
SAE generally goes by application. There's a two or more letter "service code" on most all the oil sold in a small circle. eg; "SJ". So long as that's the same or newer, generally there's compatibility. But not all motor oils are the same, in terms of additives. For instance, ND - non detergent is not useful in most modern engines. But Fred is correct, if you add a qt of current off the shelf 10W30, it's not going to matter short term whether is valvoline, shell, or store brand so long as the service numbers match.
Some brands provide better engine life than others. I generally recommend to test your oil at intervals, after 2-3K miles, so you can get a good idea how well it lasts and how much junk your engine puts in the oil and set the change based on that rather than manufacturers/oil merchants recommend because depends on how you drive and fuels in your area.
As to mixing fuel, unless one is an octane booster additive, I know of no fuels that combined offer higher octane than the sum of the two. If you fill up with 87 and top off with 93, it won't make the tank all 93. They sell after market race gas additive, octane boosters, etc. that in small quantities boosts octane. That was essentially what Tetraethyl lead did back in the day, aka leaded gas. The valve seat "lube" aspects of lead came after and is questionable to me. Most all of it went out with the exhaust, and only passed the intake for brief milli-seconds so could not be real effective. I remember more fouled valve stems back then. But gas was alot "dirtier" (ash, sulfur, etc) back then too.