Amazon Relational Database Service (Amazon RDS) Custom for Oracle now supports R6i, M6i, and T3 instances. R6i instances are designed for memory-intensive workloads with 8:1 ratio of memory to vCPU. M6i are general purpose compute instances designed to provide a balance of compute, memory, storage, and network resources with 4:1 ratio of memory to vCPU. T3 instances are ideal for database workloads with moderate CPU usage that experience temporary spikes in use.
Source:: Amazon AWS