I heard that different brands of oil will not "mix well" sometimes and can cause sudden oil burning. I just bought a used truck and don't know what brand of oil was used the last time it was changed. There's a good chance the dealer did it last, but I'm not sure. If it gets a little low, should I just change it all to make sure I don't have problems rather than add some not knowing what brand was used last? Or, does it just not matter at all?