M40 GBox

[arcturus]

Which oil should be best. gear box tend to get "sticky" when worked hard and hot. I will also use some "molly"

[Burdekin]

Originally Posted by arcturus

Which oil should be best. gear box tend to get "sticky" when worked hard and hot. I will also use some "molly"

I have read that 80/90 gear oil is similar viscosity to 20w/50 when hot. No idea if that is correct or not though.

I use 80/90 gear oil in the carb dash pot and seems to work well.

[Burdekin]

Googled: https://syntheticwarehouse.net/uncat...iring-a-75w-90

I use 20w-50 engine oil so would chose 80-90 if going for a gear oil on that basis.

[c1800]

Another option is Redline MTL, a synthetic gear oil. I have it in my M41 , works well, and has smoothed out the shifting and overall operation. Many reports from others whose experience is positive as well.

https://www.redlineoil.com/mtl-75w80-gl-4-gear-oil

If you wanted to go with something a little heavier, they have MT 85 and MT 90. I inquired directly with Redline before using it and they confirmed all three are suitable for M40/M41.

It is available in Europe.

https://www.oldhallperformance.com/r...n-distributors

[142 Guy]

I also use Red Line MTL in my M41 and it does seem to improve the operation of the synchros on my 130,000 mile gear box. Another option if the RedLine is hard to find in GB is GM synchromesh and synchromesh friction modified. For 1st gen Honda/Acura NSX owners the popular mix was 2 parts modified to 1 part regular. The NSX shared transmission oil with the limited slip diff and using 100% modified would cause early lock up of the differential. Since that won't be a problem with an M40 you could probably use 100% modified.

The North American part numbers are
https://www.gmpartsgiant.com/parts/g...-12345349.html
https://www.gmpartsgiant.com/parts/g...-12377916.html

The part numbers in GB might be different. A Chevrolet dealer may be able to help since the modified version is popular with Corvette owners with tired synchros. There are other brands of friction modified transmission oils that claim to do the same thing if you search around a bit.

Be aware that GM and other vendors sell small bottles of what are called friction modifiers. Stay away from this product as it does the opposite of what the friction modifiers do in the transmission oils. These products are intended for use in limited slip differentials and reduce friction which will reduce / eliminate the effectiveness of your synchros (synchros need some friction for them to carry out their function).