In this paper, we introduce and evaluate Selective Field Transmission (SFT), a middleware mechanism that decouples transmission content from statically defined message types in publish-subscribe systems. Industrial and robotics developers often face a dilemma: They can follow established best practices and use standard message types, such as in the Robot Operating System 2 (ROS 2) and COVESA projects, to benefit from reusable and interoperable interfaces, or they can introduce proprietary, project-specific message types tailored to receiver requirements to reduce bandwidth. SFT resolves this trade-off by dynamically adapting the transmitted message components to each receivers actual needs while preserving unmodified standard interfaces. Receivers declare or automatically derive the required message components, which are communicated to the publisher. The publisher then serializes and transmits only the required component subset per receiver with minimal developer intervention. Our evaluation shows that SFT achieves significant bandwidth reductions without measurable per-message latency overhead, with savings proportional to the number and size of unused fields.