Oil burning question

Got an older model Focus 99/00 UK model.
Obviously, its burning a little bit of oil due to its age.
Started using Castrol Magnatec, as I thought that would help slow down
further wear on the engine parts. However, this stuff is rather expensive and found I was getting through quite a lot of it, approximately 4 litres, over a 5 week period.
Switched to regular motor oil of the same grade, basically because it was cheaper. However I've found that the engine seems to be burning through this a lot slower.
Think I'm topping it up half as often now I'm using regular as I was using Magnatec.
Any reason the Magnatec would need topping up more frequently?
TIA
--
Best Wishes
Simon Taylor
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