Synthetic and regular engine oil can be mixed together. It is also possible to buy premixed oil, which may give some indication about its expected lifespan.
Synthetic and regular engine oils are similar, but synthetic mixtures tend to last much longer. Premixed oils are rated to last longer than regular oil but not as long as synthetic oil. It can be difficult to determine how long a custom mixture should be kept before it is changed, and waiting too long can have a negative effect on engine performance. Checking mixed oil on occasion can indicate if it needs to be changed.