If I remember, according to Auto tech school all SAE rated oils are Compatable chemicaly. Switching from one brand to another is not allowed to cause problems to your engine.
Talking about mixing oils at this stage of the thread seems to be off topic now.......however.....
Oils may be "chemically compatible", but the prudent way is to look at them as being "not chemically incompatible".
Which means that if you have to top up with a different brand or viscosity, it should not damage the engine. Much better than running low. But it would then be advisable not to stretch the oil change interval.
There is no way to know if your own recipe will perform as well as individual oils that have gone through extensive testing without putting this recipe through its own extensive testing as well.
And mixing the two oils mentioned by the OP and running the mix for 8000 miles without seizing the engine is not such a test!