Cant find any information that says this is true, everything I researched is showing 140. Even had to double check with a couple racing buddies who verified. They all said like a 75-140 weight for the multi viscosity. Synthetic isnt designed to be used in a transmission thats 45 years old that has never seen a rebuild which is what I am assuming is the situation with this trans is. Synthetic is designed for closer tolerances. Anything with wear and poor sealing will surely leak even worse. As would a 45 year old motor that you just started to run synthetic in. Dino fluids allow the expansion and contraction of seals better as well.

dont take my post as offensive, I just cant find any info to back this up.

 

In this day and age I wouldnt run a single weight oil. Im not sure of your location but I would think a 75w-140 would do the trick.

 

so with a 90-140 mix you get the best of both worlds?

 

Kinda its not a mix though.

Since your in Canada I would go with a 75w-140w. What this means is the oil can carry a form of multiple oil thicknesses. The idea is when the oil heats up it breaks down and becomes thinner. the breakdown and thinning is somewhat bad for parts. So as the oil heats up it takes on the form of a thicker oil up until the 140w in its design. This is the general idea on how a multi viscosity oil works. It flows like 75w but performs like 140w.

If you have a mustangs unlimited book look at page 145 bottom left corner you will see a 75w-140w oil for example.

 

Not true. Thinner oil is designed for closer tolerances in anti-friction bearings (ie. engine bearings). Synthetic oils are designed for better flow and increased stress resistance. You can apply much more pressure to synthetic oils before they squeeze out from between two surfaces, and they provide much less friction between those surfaces while they slide past eachother. In a gear and friction bearing setup, there is no downside to running synthetic oil, aside from potential leaks at the case mounting surfaces.

 

Synthetic is almost always better. Gear and trans lube can be a different story though, some trans run better on synthetics and some on minerals, but that's more an issue of the quality of oil available for the required spec. Some of the older trans are known to leak when synthetics are used, but that's just poor sealing technology, not a fluid issue. For instance, GM Synchromesh is considered to be one of the best fluids to run in TREMEC trannies(and recommended by TREMEC), and it's a mineral(not synthetic). In other cases, where gear oils are specified, there are very good synthetics available, such as Mobil 1. Amsoil does provide a synthetic Synchromesh that apparently is very good. But viscosity is viscosity, a 10-30 synthetic is the same weight as a 10-30 mineral, the difference is that synthetics typically have much higher resistances to thermal breakdown, provide better lubrication, and are more resistant to shear at higher temperatures. They also have less impurities.

As far as synthetics on the old 3 and 4 speeds though, some people say they shift great, some people say they suck at shifting. If it shifts fine and doesn't leak with a synthetic, use a synthetic if a good one is available.

 

Synthetic is ALWAYS better IMO. I run it in my cobalt and I ran it in anything rebuilt with new internals and low miles. Every time I ran synthetic in a motor and trans that had higher mileage the damn thing leaked like a siv! I had a 200 ci in my 66 mustang that leaked way bad with mobil 1, the old T5 in my turbo coupe used redline synthetic and it leaked bad, and I ran the mobil 1 in my 2.3L turbo coupe and it leaked worse lol. Once the new T5 was in it never leaked same with the cobalt using redline and mobil 1 respectively.

Starfury your right I dont know why I put that. The thinner oils are what is used today more or less as clearances are tighter. My cobalt uses 5W30 mobil 1 most wouldnt think of running that in a 45 year old motor synthetic or not.

my reasoning for NOT running it is it all ready leaks I wouldnt want to see it leak more. Ive heard tremec requiring the synchromesh GM oil, I think all richmond transmissions also require it as well or not?

 

The reason I said almost always was what was stated, they can leak. They do always lubricate better, but that doesn't make them the best choice. Great lubrication is useless if it all leaks out. And again, some of the older manuals are known for having poor shift quality with synthetics(this seems to be hit and miss though).

Believe me, I'm a fan of synthetics, and pay $10 a quart for the Torco SR1 I stick in my engine. GM synchromesh is spec'd for TREMECs and I think Richmond recommends it as well. Richmond also recommends Torco MTF for their manuals. I'm running the TORCO MTF right now in my TKO 600. Not sure what to think though, it seems a tad notchier maybe with the Torco than with some NAPA ATF. I'm gonna try the Synchromesh next and see how it fares(it may be mineral but it's some REALLY good fluid). TKO's are known for being a tad notchy as it is, so if it runs the same on the Synchromesh then I might just go back to the Torco, since I know it'll protect the hell out of the trans and it costs less. Torco's MTF is a really light viscosity manual trans fluid.

 

5W30 isn't very thin. I run 10W30 in my motor, which is only slightly thicker at startup and the same viscosity once warmed up. 5W20 is what a lot of manufacturers are switching to for reduced-tolerance, high efficiency engines. It's much thinner once heated up than 10W30 or 10W40.

 

The only reason they've gone to tighter clearance is to run thinner oils, because it takes less hp to run an oil pump that's moving solvent(that's what that w20 crap is like). It helps them squeak by here and there to meet the CAFE standards. When internal clearances allow for it, heavier oils are prefered because they provide more protection.

And on this note, this bares mentioning. Viscosity is NOT how thick the oil is. Technically, viscosity is the fluid's resistance to molecular shearing within a given temperature range. It just so happens that viscosity is very closely related to how the oil pours at certain temperatures, so that's how it's measured usually. There is sophisticated lab equipment that actually can measure shear, but it's extremely expensive so the pour method is what is typically employed for rating purposes. That's one reason why(for those of us old enough) if you look at oils today and think back to oils of yesteryear....even a 20-50 oil of today is like water compared to a 20-50 of 10 or 15 years ago. Lubricant engineers have managed to get the oils to be "thinner" and thus flow more easily, while still maintaining the same amount of shear resistance(true viscosity). That's also why synthetics of the same weight are typically a bit thinner and will leak more easily, the oil itself is thinner, but just as/more resistant to shearing forces. That's also why some companies(like Torco for instance) the 10-30 and 10-40 have nearly the same pour and flow characteristics and flow pretty much the same at temperature, but the 40 has a higher shear resistance and offers more protection for roughly the same amount of energy to run the pump. Ultimately oil weight is a measure of the oil's ability to maintain a protective film in the high heat/pressure/friction of an engine. The ideal engine oil would provide the protection strength of a 60 or 70 weight but with the flow and pumpability of a 10 or 15 weight. That's also why there's so much money locked up in good synthetic oils, it's all about providing a lubricant that will rob as little power as possible but provide the best protection at the same time. And there's a tremendous amount of technology involved, much of it in the additive packages.