Re: can you mix oil brands? -when topping up?
All present oils can be mixed; nothing harmful will happen (no sludge for example). You can even mix mineral or semi synthetic with synthetic (most of claimed full-synthetic oils have large mineral content as well).
The point is, that mixed oil , even the same grade and type may (but may not) loose balance of its additives and might not be up to the original spec. If your car is relatively close to oil change there is no problem, but I wouldnít mix two oils if I expect to drive it for thousands of miles more. (Although the two might work perfectly together, but without laboratory test you can never know)
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