The Cellular Vehicle-to-Everything (C-V2X) technology has been specified to support V2X communications in Long Term Evolution (LTE) and beyond cellular networks. Whereas Release 14 specifications mainly target the sidelink support of periodic cooperative awareness messages (CAMs) exchanged among vehicles, 3GPP is currently discussing C-V2X enhancements to enable new features in 5G Releases 16 and 17, among which the support of asynchronous messages like Decentralized Environmental Messages (DENMs). Such type of messages can be generated upon an accident or other hazardous events on the road, but also to allow a vehicle to join/leave a platoon. They need to be promptly and reliably disseminated to guarantee safe driving. In this paper, we analyse the DENM delivery performance over the sidelink, when considering the C-V2X autonomous resource allocation scheme, also known as Mode 4 in Release 14 and as Mode 2 in the next Releases. The suitability to DENMs of the resource selection algorithm specified for CAMs in Release 14 is first investigated, before analyzing some suggested parameter adjustments for Release 16, and elaborating on their most suitable tuning under different simulation settings.