I heard once you started eating Mexican food, you can no longer eat Chinese.
Does anyone know how these rumors/myths/old-wive's tales get started?
Sure, early synthetics in a high mileage vehicle caused some leaks ... but that was an early PAO formula (Mobil 1, mostly) in marginal engines which were probably prone to leaking anyway.
No idea where the reverse idea came from.
--- Bror Jace
My only issue-which I can't really prove and have mentioned before: Each oil will be slightly different as far as seal swell goes. I think that when you change oils the oil used previously will be replaced by the new oil. I think this eventually will cause a seal to leak. Just an opinion though.
I have mixed synthetics and dino (making my own "blended" oil) frequently in multiple vehicles (two Nissans, a Ford and a Dodge, all with over 100K on the clock) with absolutely no problems. I am not recommending this for you, but just to let you know it has worked for me.