Well, I am in the same boat. Have run Delo 400 CI4 for 200k with no issues. Now due a change and not sure which way I am going. Had a chance a while back to get some Premium Blue Classic, which is CJ4, but is no longer available. Thinking of trying the Premium Blue anyway. Don't want to go synthetic. Nothing against it, just my thinking for the present. Post what you decide to use.