RL oils are primarily PAO based and their current reformulated street oils probably contain no more than 15% POE.
All PAO based oils contain some esters.
Adding a quart of RL to a GP III synthetic isn't going to add much ester but as you pointed out you would be likely be increasing the moly level but what you will be really increasing is the ZDDP level which is very high in RL oils.
So the primary benefit I can see in adding some RL would be to increase the ZDDP level of an OTC GP III oil if you think the engine would benefit from a higher AW level, that primarily being increased ZDDP.
BTW, my favourite RL grades are 0W-20, 0W-40 and 5W-50 for blending purposes.