You can buy a 55 gallon drum and it saves you some money. You have too get a pump and a pouring can but in the long run it makes things easier.
As far as synthetic? in a diesel I think it is a waste due too not being able too extend oil change intervals due too contamination.
Brand? no way too prove one is better than the other. If it has the same cert it will be equal in ability.
Gas engines I use Mobil synthetic with oil change intervals in the 15k mile range. Have multiple vehicles with 300k plus miles and no problems. Even had the oil tested with 15k miles on it. It still tested better than requirments for new oil.