Carol your medinillias are gorgeous and they look like they have gotten huge. I lost a scortechinii last winter to the cold, it was my own fault, I thought it was in a protected spot but last winter broke all the rules.
Your Big R is probably a regional, west coast, product. Around here the growers use triple ground bark in their mix along with sand and perlite. The bark is likely a combination of pine and hardwood, whatever is available.
I have been using a product called Perma Til in my soilless mix, it is expanded shale and it really improves the drainage and because it is heavy like gravel, it adds some weight so the pots don't fll over as easily in the wind.
