I'm running approximately 15 year old conventional Citgo Supergard 10w40 in my '16 Versa. It just hit 30K miles and I did a transmission fluid drain/fill/filters. It will be due for an OC in about 1K miles and was wondering if since I have a few jugs of 15 year old 10w30 Pennzoil Platinum in the garage whether the members of the forum thought it would be beneficial to run synthetic oil for one OCI to help clean the engine then go back to the conventional or would you just continue to run the conventional? I know synthetic is supposed to be better oil but I've got probably around 8-10 cases of conventional that needs to be used up so I can't afford to just let it sit and switch to synthetic full time. I've ran conventional 10w40 for 45 years without any issues. Will the synthetic actually do any more cleaning than the conventional? I don't know much about oil technology thus the reason for the question.