Originally Posted By: Wampahoofus
I'd like to pick your collective brains. I picked up a Chrysler 300M with the 42LE automatic in it. It has a transmission and hypoid final drive in a common case, but with separate fluid reservoirs for each. The owners manual states specifically "use 80w-90 GL5 gear lube in the differential.
Synthetic lubricants should be avoided."
The gear lube and ATF reservoirs are separated by 2 internal seals (if both fail, they can mix).
I'm guessing the 'no synthetic lubes' edict comes from Chrysler believing that synthetic lubes are more likely to leak, and this is just a CYA move. Is there more to this that I'm not picking up on?
I humbly don't think so.
In a proper design of such a system, a drain hole at 6 o'clock position is normally provided .......
a) to drain off leaked ATF oil exiting from oil seal on the LHS, and
b) to drain off leaked differential gear oils exiting from oil seal on the RHS. A full disassembly shall confirm this design, whose aim is to prevent undesired mixing of different oil types.
A mineral oil of 80W90 by 'nature', shall (post 2000's) have KV@40*C of 135-165 cSt and KV@100*C of 13.5-18.5 cSt. A synthetic "won't" be able to achieve this and it may not make marketing sense to the businesses.
For a synthetic to conform to SAE xxW90 gear oil grades ...... with primarily KV@100*C of 13.5-18.5 cSt which is non-negotiable in a way as far as SAE/ASTM are concerned, things were done resulting in say, a 75W90 oil grades
whose KV@40*C would drops ........ way, way below minimum 135 cSt KV@40*C of typical 80W90 prevalent then.
I suspect some OEM manufacturers do realise this new formualation of say,
75W90 would result in drop in differential gear oil operating viscosity at an operating temperature they know best (which I would hazard a guess at 55*C-65*C depending on specific applications). Not all OEM manufacturers are worried/bothered about this phenomenon.
But some OEM manufacturers do , realising detrimental effects of lower operating viscosity on their equipments ..... thus out go the so-called recommendation of "Synthetic lubricants should be avoided". This I call a
conscientious manufacturer, ruled by engineers, NOT by a bunch of business beings.
Originally Posted By: Wampahoofus
FWIW, I drained and filled with 80w-90. I'd really like to use a synthetic 75w-90 since I live in a winter wonderland, but even more than that I don't want to rebuild a transmission.
If you really like synthetic, go for 75W110 grades that offers quite similar level of components protection as a mineral 80W90 do.
Heck, why not follow Ford's 75W140/80W140 recommendation!
Just my 2 cents.
