I hv never drive lower than the mileage when compare with the mths (whichever come 1st), tt means i ll need to service once mileage achieved 10K (using fully syn) usually ~ 3mths time (therefore not sure if wait till 6 mths or more).
My experience, did service few times ~12K (more than the recommended 10K), fully service at TC since day 1, only saw some comments on their service reports to "service on time".. other than that, not an issue.. therefore still can drag on over the recommended mileage interval..
As for you 3 mths (using normal oil), or 6 mths (using fully syn oil), would recommend u to go servicing on time, maybe slightly more up to 1 mth shd b ok..