Which is the trap OID for syncfailed

There's no OID that could run a test and tell you if it would work well at that moment, but the best thing you could do is get the sync status between the two devices. There are a few OID's to look at for that.

These are from the system MIB

snmpwalk -c public localhost .1.3.6.1.4.1.3375 |grep sysCmSyncStatus
F5-BIGIP-SYSTEM-MIB::sysCmSyncStatusId.0 = INTEGER: standalone(6)
F5-BIGIP-SYSTEM-MIB::sysCmSyncStatusStatus.0 = STRING: Standalone
F5-BIGIP-SYSTEM-MIB::sysCmSyncStatusColor.0 = INTEGER: green(0)
F5-BIGIP-SYSTEM-MIB::sysCmSyncStatusSummary.0 = STRING: 
F5-BIGIP-SYSTEM-MIB::sysCmSyncStatusDetailsNumber.0 = INTEGER: 1
F5-BIGIP-SYSTEM-MIB::sysCmSyncStatusDetailsIndex.1 = INTEGER: 1
F5-BIGIP-SYSTEM-MIB::sysCmSyncStatusDetailsDetails.1 = STRING: Optional action: Add a device to the trust domain

This is the output from a standalone so it doesn't show exactly what you need, but you're looking for the status to be green.