some of the orings in these rc engines we use don’t do well with petroleum products I would read what the manufacturer says. with that said I have always added some castor to my synthetic oil based fuels to bring up the oil content to about 20% and before synthetic oils there was castor. if I am planning to store an engine for a season then I will use some type of oil in the engine usually atf.the only problem I have ever had with castor oil is it tends to gum and makes a mess other than that I recommend the use of it.i have motors that are over 30 years old and still going strong. I attribute it to the fuels/oils,maitence and proper storage. friction and heat is one of the main causes of early engine failure. and these new engines with their ball bearings and a lot with abs construction and high rpms a little extra insurance never hurts. day to day flying I would say you don’t need any after run oil, I always use it but you shouldn’t have any problems. for storage over a few months I would use something atf.marvel oil, after run oil.