Many oil companies are making outrageous claims about there oil, that I disagree with. Change your oil once a year, if you only drive your vehicle on long trips and not to often it may work for you, but if you drive normally in and out of town, it will shorten the life of your engine. Semi synthetic, many are less then 5% synthetic, Cheap oils from any auto parts store, not worth the money and cheap oil is cheap oil. Oil for high mileage vehicles MAY be worth the extra $ but I am not quite convinced.
If your vehicle is new use a quality synthetic, I use Mobil 1, or used car use a high-quality oil. I use Mobil 1 in my pickup and my wife’s car and Mobil extended performance in my 2 older cars.
Changing to our from synthetic is not recommended, you may end up with leaks or problems from sludge that synthetic can break loose. Synthetic to a regular oil is OK but I see no reason except for cost.
Changing the viscosity of your oil in a high mileage engine IE from 5-30 which is recommended to a 10-30. I really don’t have a solid answer on this one as it depends upon the engine. Many have upper cam problems, due to close tolerances. Changing to a 5-30 in extremely cold weather is OK. I switch my pickup.