Right way to make cake batter that asks for butter and oil
You probably won't like this answer, but:

Do what your recipe tells you to do.  

There are various methods to combine a set of ingredients and each will lead to a slightly different outcome. Drier or moister, lighter or denser...
The "right" result will depend on the expectation of the recipe writer and can be the root of an eternal discussion: Compare the cookie lovers that will forever debate about cakey vs. crumbly, soft vs. crisp... and if they should ever agree (highly unlikely!), I'm sure the brownie fraction will fill the gap with a similar debate.

So unless you want to tweak a specific recipe, the best advice I can give you is to stick with your current instructions, as they should also match the ratios of the given ingredients (another factor that should not be ignored). If the results are not to your liking, try perhaps a recipe with another method and/or post a more specific question. 

And one anecdote that illustrates how much the "right" method can be misleading:
My mom, experienced baker and all that, kept wondering why one of her cakes came out only half as high as when other family members made them. (And way less experienced bakers to boot!) Once she stopped mixing the ingredients "the right way" - beating the egg whites separately and folding them in at the end - and followed the simpler instructions (which she had ignored before) of using whole eggs, she finally got the desired results.

So in short, there is no "right" method that fits all.