Interesting but difficult question due to crude oil is a complex mixture of hydrocarbons as aliphatic, aromatic, asphaltenes etc... even composition between different type of crude oil i.e. crude maya oil or west texan is no the same. Maybe you can make an "model crude oil" by the mixture of aliphatic, aromatic and asphaltenes standards but fixed one type or oil as a reference.
The question is simple but the solution may not be that simple. Depending on type crude oil you need (or for what purpose the "synthetic"crude oil is needed), it is all about mixing just ingredients. Hereby 2 quick links with some information about crude oil types.
Your question is quite complex. I recommend you start designing the crude starting from a mixture of products that have high content of saturated and aromatic fractions, and then subjecting this mixture to thermal cracking reactions in Batch type reactors and an inert atmosphere between 250-500 psig of Nitrogen. You should create a training matrix so that you can vary the temperature and residence time conditions, since they are the variables that affect the quality and performance of thermal conversion products. Also I present one of my articles to have a better idea about this type of process (thermal cracking) and characterization and see if you can apply, this in my profile. Construction of a formation matrix for the production of anode-grade petroleum tar pitch by thermal cracking. Once you obtain a viscous product you can use ASTM D287 - 12b Standard Test Method for API Gravity of Crude Petroleum and Petroleum Products (Hydrometer Method)