In our lab, we simply mix crude oil with light pure hydrocarbon such as decane. However, this can bring some difference in other chemical and physical properties. If there are large amount of heavy and polar components in the crude oil, be careful in using too light hydrocarbon such as C6-C8.
Yes, temperature tratment is usefull, also very small amountl less than 500 ppm of polymer additives such as depressants are very effective but it is temporary effect...