I would look at the rat genetics - there is a lot done (10.1016/j.tig.2006.02.009).
Not sure if you intend to use Lewis rats for studies on organ preservation and transplantation, but if so one should bear in mind that some of the most effective organ preservation media contain dextran which unfortunately elicits direct histamine release from mast cells in almost all species of rat ( except Whistar -NR or NELP strains). This means that results may be difficult to interpret when rats are used. Other rodent species (mice, hamsters, gerbils, etc ) and other mammals, incl man do not react in this way so can be used without problem in studies involving organ preservation in media containing dextran. Apart from this caveat, dextran based preservation media such as Perfadex , etc, are superior to most others in both exptl and clinical studies, particularly in lung, pancreas and vascular tissue preservation. This rat-specific reaction to dextran has been well documented since the 1960s ...
To design your transplant experiment you want to look into the RT1 match or mismatch combination that you would desire depending on the type of hypothesis to be tested. SD rats are widely used in transplantation and certainly a good option. They are somewhat easier to handle than other strains due the fact that they are very calm.