In the old days I would settle on a brand and run it in all my vehicles. Usually the higher end product, e.g. Castrol Edge/Magnatec. I would use the leftovers as top ups. That way one vehicle that used no oil could have it's leftovers used as a top up for the other vehicle that used more. I wasn't too worried if the oil weight didn't match perfectly. They were often quite close anyway.
I would also buy a cheap 5L bottle of the base oil for that brand, e.g. Castrol GTX/XL. This is what I used when I was out of leftovers.
My logic was that because all the oil came from the same company, then the additive package would be similar and work well together.
However, now days I chase sales, and my brands are all over the place. Not sure if I have gone forwards or backwards.
I would also buy a cheap 5L bottle of the base oil for that brand, e.g. Castrol GTX/XL. This is what I used when I was out of leftovers.
My logic was that because all the oil came from the same company, then the additive package would be similar and work well together.
However, now days I chase sales, and my brands are all over the place. Not sure if I have gone forwards or backwards.