I'm also a big fan of Rotella-T, and used to use 15w40 exclusively in my Defender. Those old Rover 3.5/3.9 V-8's were very dirty running engines due to a design flaw in the head gasket design, and the additional detergents in the Rotella sure came in handy.
Personally, I wouldn't worry about synthetic at this point. The main gripe I have with synthetic in older engines is that if you have any leaks, it gets expensive to keep topping the stuff off. That, and if your engine is already approaching time for a rebuild, then you won't benefit from the advertised extended engine lifespan.
-Hans
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