Originally Posted By: Benito
... Synthetic oils clean better and can remove false seals. ...
Meeeehhhhhhh .... Not really.
That is certainly the preconceived notion out in public, but we here at BITOG ought to all know better. I'm going to take exception to this and define it in greater detail.
First, we have to accept that in this case, "syns" are referring to PAO, and not group III or V, just for the sake of this particular thread. I am limiting my comments as such in this context.
PAOs most certainly do resist oxidizing better than conventional stocks. I agree with that. So it might be fair to say that they "run cleaner". But I disagree that they "clean better". PAOs don't hold additives in suspension nearly as well as conventional base stocks; that is one of their limitations. Therefore they cannot "clean" better than a good quality dino HDEO with a robust additive package. Now, PAOs finished products have been modified to accept detergents in a meaningful way, but that does not give them an ability to "clean" any "better" than a dino with those same additives. Simply put, PAOs "run cleaner", but they don't necessarily "clean better". Therefore I find it a bit misleading to state that quote above.
Presuming this "new to him" 7.3L PSD was well cared for, and had regular OCIs, there's probably very little in the way of sludge/deposits on the seals, or anywhere else for that matter. Therefore you can't claim to be able to:
1) clean better when PAOs are limited in this regard contrasted to a robust dino
2) clean something that isn't likely there in the first place
I want to be clear that I am not a chemist, so I cannot explain all the intricate details of the elemental aspects of this topic. But after conversations with those who do fill that bill, this is probably a more accurate statement that the quote above.
Now, it was also true that older formulations of PAOs did react with some older seal materials, and actually cause the seals to shrink. But as I stated previously in this thread, those two topics have been addressed by the industry long ago. PAOs now have modified seal swellers, as I understand, as well as most automotive seals have been compatible for a long time now.
Therefore, we have to be careful when we talk about seal leaks in the first place.
Does the seal leak because of mechanical abrasion? (not the fault of any lube)
Does the seal leak because it hardened? (either chemical or heat induced)
Does the seal leak because of shrinking? (chemically induced)
It is possible, given the age of that '97 PDS, that some seals may be hardened by heat or old enough that material compatibility may be an issue. However, given that this PSD was claimed to be in good shape and had regular OCIs, plus he's using Amsoil, which is a very capable product line, and there's no leaks currently, I'd say he's OK to at least try it.
Part of the problem for this OP is that his trusted mechanic threw a blanket statement over a very detailed topic. And he is probably operating on old mentality when it comes to seal leaks using syns.