I do not claim some expertise on the issues of trans fluids, and know this board has some really great brains on this subject. So, with that warning let me offer what I *understand* to be the case, and pray someone a whole lot smarter than me reads it and chimes in (possibly telling you the following "advice" is FOS.)
Merc V is s semi syn. I think almost by defeinition that means it offers some superior anti oxidizing properties and will handle and disperse heat better with less problem sludges and varnish. However (starting onto thin ice here) it is also my understanding that the friction properties in the additives in Merc V make it perform differently than Merc III - and that the friction materials in the Merc V trannies is formulated slightly differently as a result.
IF that is correct, I would not just adjust the diet of an old trannie with frictions in clutches, bands and the TC made for Merc III to start using Merc V.
If you are interested in the improved qualities synthetics offer I'd go full syn... mind you it will be pricey.
Not sure if this helps you or not. Take the italicized advice with a grain of salt until someone confirms this.
(ps. ALL too often someone on this board lacking real knowledge will offer as "gospel" some advice that is just plain flat out wrong. I suggest you 'confirm" advice you receive and be aware of the qualifications and reputation of the person offering it.)
