75w90 vs 80w90 915 trans

[wearl]

will using 75w90 in 915 trans make my 84 911 shift better when cold vs 80w90.

[Jerome74911S]

Try it and tell us.

[Peter Zimmermann]

Originally Posted by wearl

will using 75w90 in 915 trans make my 84 911 shift better when cold vs 80w90.

In the Kendall (my recommendation) system 75w/90 is synthetic gear oil - you do not want that. 80w/90 is referred to as NS-MP, and is a dino gear oil that's perfect for your car's 915 transmission.

[Jerome74911S]

Sorry about my rude comment, above. No offence meant.

Pete Z's remarks are logical and consistent with accumulated wisdom about the way 915 transmissions function.

It's puzzling to me, though, that my 915 transmission shifted smooth as silk with Amsoil synthetic, and now that I have changed to Kendall NS-MP, the shifting is no different. Still nice and smooth, but not better.

[Peter Zimmermann]

Originally Posted by Jerome74911S

It's puzzling to me, though, that my 915 transmission shifted smooth as silk with Amsoil synthetic, and now that I have changed to Kendall NS-MP, the shifting is no different. Still nice and smooth, but not better.

The synthetic that you were using will deliver good (very good) cold shifting, but...synthetic oil generally provides too much "slippery" to the friction-based synchros used in the 915. This will eventually shorten synchro life-expectancy when compared with a good dino oil like Kendall.

It really makes a pro-Kendall statement when you can say that your 915 shifts as well, with Kendall dino oil, as it did with Amsoil! The difference is that the Kendall oil will allow good shifting for many thousands more miles.

Kudos to you for probably using correct driving techniques as well!

[Jerome74911S]

Good answer. Thanks.

[wearl]

i put new bushings on my shifter,shift rod, and coupling and i put valvoline high performance non synthetic 80w90 in the trans. when car is warm it shifts great, but shifts a LITTLE STIFF WHEN COLD. valvoline also makes a 75w90 in non synthetic. that is why i asked question about it possibly working better when trans is no warmed up . also the manual says 75w90 . i did not read that until i had put in 80w90. the reason i went with valvoline is because i can get it without a hassle at local parts store.

[RichardM912]

A conventional mineral oil-based gear oil (that is, a non-synthetic) can achieve the SAE80W/90 viscosity specification without the need to add any Viscosity Index Improver. However it is not possible to achieve the 75W/90 specification.

To do this you either have to use some (or all) synthetic base oil or add some Viscosity Index Improver. These latter materials are polymers and they have been used for years to formulate multigrade engine oils (again, before the arrival of synthetics).

The problem is that a gear box is a much higher shear environment for the oil than an engine is. It is hard (and expensive) to develop a polymer that will survive intact in a gear box.

This is why wider multigrades such as 75W/90 have not been generally available until recently. This coincides with the wider availablity and lowering of costs of synthetic base oils. Most of these products are partly or fully synthetic.

However, it is certainly possible today to formulate a 75W/90 using conventional mineral oil if you choose the right Viscosity Index Improver. If you plan to purchase such a product you should attempt to check its shear stability. I am not aware of any adequate US specifications for this property but there are European ones. There is a good VW test and specification. I would check the Valvoline label fine print where it may list specifications or ask Valvoline for info.

Synthetics do not have this shear stability problem (which results in a drop of viscosity). They, and conventional mineral oils, are completely shear resistant.

I will leave it to others on the relationship between synthetics and syncro performance. However, a 75W/90 will give easier cold shift (and actually a little higher viscosity at high temperatures (over 100C/212F). These are oil temperatures, not ambients.

[DRACO A5OG]

Originally Posted by Jerome74911S

Good answer. Thanks.

If I may add I had to change twice to flush the old stuff out. I have OCD so I did three times after 1000 miles each. (PZ thought that was nutz)

With a new Derlin Shift Coupler, Sienes Gate Shift Kit, Cab Motor and Engine Mounts, she shifts like a dream, Smooth and Precise as it should be

Thanks PZ

[FenderGuy]

Is Swepco no longer recommened by the board? Swepco Transmission Fluid Question

Thats what I had in my 915 improved my shifting when I had it in my tranny

[winders]

Originally Posted by FenderGuy

Is Swepco no longer recommened by the board? Swepco Transmission Fluid Question

Thats what I had in my 915 improved my shifting when I had it in my tranny

Swepco 201 is an excellent choice for a 915 transmission. Don't let anyone tell you any different.

Scott