What is wrong with it? Egads! Idiots are everywhere.
Mixing brands alone is dumb. It is ok if you Switch brands, and make sure you get all the old oil out of the engine. When you mix brands of oil in a engine, you turn it in to a slot machine. The pay off is rods thru the block.
What happens? First the different detergent packages are not compatible. Then second your mixing crude grades. You can mix like grades (such as Pennsylvania grades, but you can't mix then with say a Texas grade crude). Mixing Crude grades will just cause sludge, and problems.
As for weights, mixing Multi-Viscosity weight oils is stupid. Not only do they react to tempitures differently, they will separate in the oil pan. The thinner oil will sit a top of the thicker (5W30 vs. 10W40) oil.
While your at it you might want to add tome Slick 50 to the mix. That way you also have 50 weight oil with PTFE floating around to clog your filter, and bearings. That way you can drop another engine in sooner. Charles