Had my truck manual transmission rebuilt by a well known shop and noticed they used a black RTV. I contacted the shop and they said they use the Locktite brand. Further research showed that Locktite makes this high heat oil resistant version. I used it to add a cooler to the PTO cover and to seal the transfer case to the transmission. I guess only time will tell once I get my truck on the road.
RTV 598 worked great to seal a small gap in a high-temperature heating device. Highly recommended.
Note that due to its viscosity, it takes a lot of force to squeeze out a bead less than 3/16". A larger bead will take less force to apply.