I would never use semi-synthetic oil or synthetic blend. Here in the United States, there are no regulations that specify what the percentage of synthetic oil is in the blend. So, one could purchase a semi-synthetic or synthetic blend that is less than 5% synthetic and there is no way of knowing as the percentages are not required to appear on the label. And although it may be true that even a small amount of synthetic oil added to mineral oil has some benefit, there are fully synthetic oils that are only slightly more expensive than the semi-synthetic or synthetic blends. So I will use full synthetic in my cars.
I would never use semi-synthetic oil or synthetic blend. Here in the United States, there are no regulations that specify what the percentage of synthetic oil is in the blend. So, one could purchase a semi-synthetic or synthetic blend that is less than 5% synthetic and there is no way of knowing as the percentages are not required to appear on the label. And although it may be true that even a small amount of synthetic oil added to mineral oil has some benefit, there are fully synthetic oils that are only slightly more expensive than the semi-synthetic or synthetic blends. So I will use full synthetic in my cars.