75w-90 or 75w-140?

[E21BIMMERHEAD]

any thoughts on using 75w-140 instead of 75w-90? Im going to put in some synthetic stuff(brand preference? grassroots motorsports mag suggested somthing that I cant remember right now cuz im tired...) I dunno I put 10w-40 in over the hot parts of summer and have had wonderful success with it, so why not try the same thing with the gearbox oil? the only thing I can think of as why I should not put 75w-140 in is because its beginning to get cold and well hell it takes forever in MN for the gearbox to warm up with 75w90. Im thinkin it will never warm up with 140 but at the same rate 140 weight might help if I get stuck and need to do some rockin'....? thanks again for your input!

 

[protege321]

75w-90 is what is recommended by mazda. just my opinion, switching to synthetic is a bad idea, your gonna have shot seals in 10k miles. and honestly synthetic or conventional.. are you gonna notice a difference? go with whats recommended.

 

[E21BIMMERHEAD]

75w-90 is what is recommended by mazda. just my opinion, switching to synthetic is a bad idea, your gonna have shot seals in 10k miles. and honestly synthetic or conventional.. are you gonna notice a difference? go with whats recommended.

I hear ya on the whats recommended and all, but Iv been pondering the synthetic partly because Mt-90 by redline and valvolines synthietic stuff both claim to recondition old seals.....I believe my driver side axel seal is leaking, already replaced the tranny once and dont want to again......Now Im kinda thinking that maybe changing the fluids to some that "recondition old seals" may help this out(Im goin to try to find time to take some pics to post up and show ya). The leak is mearly slight at best, so I guess im figuring try different fluids or just keep checking the level and topping off as needed while I gather $$ and parts to do something beefier. F/G hybrid with MSP LSD and axels??

 

[Inovagent]

For synthetic oils it's all about pour point. Even an identically rated conventional oil is going to be like sludge when it's really cold out. The benefit of the synthetic oils is that shifting will be smoother on those really cold mornings.

There are other advantages like film strength, durability, resistance to oxidation under high heat conditions, etc but most street cars aren't going to be subjected to the kinds of conditions where those will make a difference.