No way you should be using fully synthetic in a 130 carburetor engine.
Best oil is Shell Helix 15w-40, the yellow gallon.
Castrol GTX 20w-50 will just cause sludge in the sump and if you have been using that oil before, I suggest you get the sump cleaned out or else you maye end up with worn out valve stem seals and rings at worst. Magnetec on the other hand is just too weak and it will push through the seals and gaskets.
Fully synthetic in a 1.3 carb is fine. It may just be costly compared to other options. Fully synthetic doesn't denature as easily as mineral or semi synthetic oils.
I don't agree with what was said about the Castrol oils. GTX is the budget 20w50 oil from Castrol. Never seen it cause sludge. Magnetic is slightly thinner than GTX at 10w40 and claims to stick to the surfaces better, not sure if that's true but it seemed to be better than GTX at cold start.
If any oil is pushing through gaskets, the gaskets need replacing. It's not the oils fault. Any similar weight oil will do the same thing.
Never add any additives or engine cleaners into the oil ever, these cause sludge.
Just change your oil and oil filter at the service intervals and your car will be fine.