500ci Oil Question

[dannygila]

Hello All,
Would Castrol high mileage synthetic blend be safe or even recommended for a 76 Fleetwood Brougham 500ci with 87,000 miles, or should conventional (10w40, 10w30, or 15w40) motor oil be used? I am unsure if the previous owner used conventional or synthetic, but was told if synthetic was used, I  shouldn't go back to conventional, but to be safe, use a blend. Not sure what to do. Thanks for any input.
-Dan

[TJ Hopland]

Back in the 70's when synthetic oil first came out there were possible issues mixing it but those were quite different oils than what is around today.  We are not running these engines that hard or that much so they really are not that picky.  If you have had good luck with or have people you trust recommending the Castrol go for it. 

Where the bigger drama comes in these days is the whole zinc thing.  Personally I don't think its a big deal for these engines and the way we typically use them but others feel quite strongly the other way and use additives and or oils that claim to be specially formulated. Plenty of threads here and every other car and engine site if you want to read up on the subject.   

[hornetball]

I've been using Rotella full synthetic in mine and it runs nicely.  The reason I'm using the diesel oil is that when I went into the motor to replace the timing chain, it was pretty gunky.  I'm hoping the additional detergents in the diesel oil will help clean up the motor.  You would never know it though, it's smooth, strong and quiet -- gobs of torque.