RED LINE MT-90 HIGH-PERFORMANCE GEAR OIL is a fully-synthetic ester-based lubricant designed for use in manual transmissions and front-wheel-drive transaxles. This shear-stable formula resists breakdown for extended periods and offers more consistent viscosity in warm and cold temperatures for less gear noise and improved shifting. This fluid is less slippery for improved synchronizer coefficient of friction and offers stronger film strength for better wear protection.

Red line MT90 is a synthetic 75W90oil designed for use in synchromesh gearboxes. It provides excellent protection to gears and improves the life of synchro rings. It has a perfect coefficient of friction to allow easier and faster gear changes even when cold. Where I live I'm a fan of MT90. But if you drive your car when it's cold outside MTL would likely be the best choice. If you have any doubts just call Redline-- I'm sure they'd happily advise you. In any case, you won't be regret installing either MTL or MT90. High performance gear protection giving longer synchro life and eliminating notchy shifting, even when cold. Satisifies the gear oil viscosity requirements of 75W, 80W and motor oil viscosities of SAE 40, 10W40, and 15W40. Recommended for GL-1, GL-3, and GL-4 applications, as well as where most special synchromesh fluids are specified. Appropriate coefficient of friction for most manual transmission synchronizers (other's synthetic gear oils are often too slippery for proper synchro engagement). https://cdn.opieoils.co.uk/images/variants/icon/redline/red-line-mt90-75w-90-gl4-synthetic-manual-gear-oil-1-us-quart-0946-litre-2.jpg Bob... That problem that you describe, " when hot, o/drive doesn't engage " is exactly what I had experienced with regular 30w motor oil.https://cdn.opieoils.co.uk/images/variants/large/redline/red-line-mt90-75w-90-gl4-synthetic-manual-gear-oil-1-us-quart-0946-litre-3.jpg

Maybe I could do a 50/50 mix of MTL and MT90, which would give me a viscocity more or less in the middle ... the best of both worlds ... ?This Gear Oil is suitable and popular for most RX-8 and RX-7 Transmissions. MT-90 75W90 GL-4 GEAR OIL RED LINE MT-90 is a synthetic manual transmission gear oil that provides outstanding protection for manual transmissions. Perfect for vehicles with notoriously difficult transmissions, such as certain Nissan, Toyota, and Mazda cars, RED LINE’s MT-90 gear oil solves the problem of hard gear shifting and allows for a smoother manual driving experience, even in cold conditions. This is the perfect 75W-90 gear oil for drivers seeking a solution to a difficult transmission that also protects and prolongs the lifespan of their gearbox synchronizers. https://cdn.opieoils.co.uk/images/variants/icon/redline/red-line-mt90-75w-90-gl4-synthetic-manual-gear-oil-1-us-gallon-378-litres-1.jpg

OK, I just went out to the garage and pulled my old blue binder factory service manual down off the shelf. not trying to justify a switch away from the use of Redline here ... just wondering out loud as to why Redline has earned such a fine reputation).

https://cdn.opieoils.co.uk/images/variants/large/redline/red-line-mt90-75w-90-gl4-synthetic-manual-gear-oil-1-us-quart-0946-litre-2.jpg https://cdn.opieoils.co.uk/images/variants/icon/redline/red-line-mt90-75w-90-gl4-synthetic-manual-gear-oil-1-us-quart-0946-litre-3.jpg https://cdn.opieoils.co.uk/images/variants/medium/redline/red-line-mt90-75w-90-gl4-synthetic-manual-gear-oil-1-us-gallon-378-litres-1.jpg