Because the oil will have a different viscosity than either oil, and they might not combine correctly. It may work okay, or it might not.
If the oil doesn't work right, it may burn, smoke, or damage the engine.
Different applications and engines require different weights of oil to operate properly and prevent engine wear. Therefore different weights of oil are produced.
As long as it is the same grade of motor oil the brand name should not matter.
weights 20 through 40 shock oils
15w20 is slightly thicker thand 10w40, .... but neither oil is used that often any more due to 5w30 and 10W30 motor oils. Unless you have a product purchased in the past five years and the manufacturer specifically calls for it .... use the other two weights I have recommended
They're re-refined.
Pennzoil and Quaker State
Conventional motor oils have recommended 3,000-mile drain intervals. Synthetics motor oils have recommend 5,000 - 25,000-mile drain intervals.
The competition is tight and in general the premium oils that you buy are mostly quite similar, with two exceptions. Traditional oils that you buy have historically been the standard with Pennsylvania grade crude making what was considered to be the best motor oils with Penzoil and Quaker State being two examples. With modern processing of motor oils most of the premium oils will serve well provided that they have the engineering certifications on the can. The other category of oils far exceeds the quality of the traditional oils. The fully synthetic oils last far longer and give much better protection than the traditional oils. Synthetics produce virtually NO paraffin as they break down. Paraffin becomes the deposits that build up on the inside of the engine and often causes problems. FULLY synthetic oils last longer, protect better and run cleaner. Engines using FULLY synthetic oils hold together far better.
you cant really remove them because they are already separated
You might be thinking of Quaker State and Pennzoil.
No, but find out the requirements for the motor. Most normal motor oils don't meet manufactures requirements for wear protection.
Few Years Back A Test Was Given On Top Selling Motor Oils. Castrol Came Out Number One On The Test. I Saw The Results And List Of Oils. The One I Was Using Came In A Close Second. So I Changed To Castrol Motor It Rated Number One. Don`t Want To Down Play Other Motor Oils. There Is A Bunch Of Great Oils That Work To The Extreme But For Me It Is Castrol.