To modify the specific properties 'N' no of additives(Chemicals) are there. for example, to reduce the viscosity of crude oil pourpoint depressants are there. also to increase the octane number (gasoline) MTBE (Methyl tertiary butyl ether) can be added.
Depends the petroleum characteristics that you will be simulate, but if you want an average characteristic fluid you can buy a commercial hydrocarbons (Hexane, decane, etc), all the alcanes have the sames properties and you can use in your experimental test, if you need more information check my profile and contact me.