25w60 is the strangest grade I've ever heard of. Where did you even find out about that? Everybody on the planet runs very conventional grades like 10w30 and so on and has ZERO problems.
Back in the day you could get Castol in a 10w50 but it was almost double a 20w50. I loved that oil but most didn't because they didn't want to pay a buck seventy five a quart!
BTW, Torco makes a 20w60 full synthetic. It's designed for some European engine IIRC. I almost used it and then said screw it and closed up my clearances so I can run 5w30 or maybe even 0w20. It expensive. $23.00 per quart.
