Hate to ask...but unless the inside of a motor looks like the inside of a kitchen oven, after ten years of use......the anti sludge oil shouldn't be a problem.
I remember, when the stuff started to be made, 60's I think, the advise was to not use it in an old motor, because the thick crap you see inside the valve cover of a car that hasn't had an oil change, just been topped up, for ten years; will wash off the walls and end up in bearings, etc. But if you have been doing oil changes, and the inside of the motor isn't filled with dead goldfish, the anti-sludge oil will just keep it cleaner and certainly not do any damage to a clean engine. If you don't know what is in the engine and you think some PO hadn't given it an oil change in 50,000 miles...then be a bit cautious. I have always done the changes before thousand miles or so, and mine is clean...and has 120,000 on it. But even mine had a lot of goldfish in the oil tank. I was amazed...leaves, screws.....a baracuda.
Would at least, remove the oil tank, and flush it out good. I didn't even know there was such a thing today, as non-detergent oil...I use Castrol too, but this won't make me change my oil brand...will check the old bottles, bet it's detergent oil too...and has been for years.
As to straight oil.....not all of us live in a climate that has a constant temperature......might be fine to run 50 in Queensland, but in the rest of our areas....the "norton leg" is unwelcome. I'd rather run multigrade.
Main point, after all is said and done, might be not the type of oil, but the frequency of the oil changes and maintainance.... :wink: