https://cdn.opieoils.co.uk/images/variants/medium/redline/red-line-mt90-75w-90-gl4-synthetic-manual-gear-oil-1-us-gallon-378-litres-2.jpg Appropriate coefficient of friction for most manual transmission synchronisers (other’s synthetic gear oils are often too slippery for proper synchro engagement) IIRC, Redline says flat out that the only difference between MTL and MT90 is the viscosity/intended temp range. Like Steve says, if the car is driven hard or lives in a hot climate, MT90 is prolly a better choice. But frankly, we've run MTL in BMW race cars where the trans tunnel actually gets too hot to touch in the summer, and never seen a problem.

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. I found MTL around 20 years ago while trying to solve some crunchy gear engagement problems in a BMW. It solved the problem so well that I've been a Redline fan ever since. It ain't snake oil. It's an engineered product. RED LINE MT-90 HIGH-PERFORMANCE GEAR OIL is a full-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. https://cdn.opieoils.co.uk/images/variants/medium/redline/red-line-mt90-75w-90-gl4-synthetic-manual-gear-oil-1-us-quart-0946-litre-1.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.I think that Dave said MT90 is somewhat equivalent in viscosity to 40w motor oil. If your o/drive starts acting up again when it gets real hot, try switching to 60w valvoline VR1. 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 https://cdn.opieoils.co.uk/images/variants/icon/redline/red-line-mt90-75w-90-gl4-synthetic-manual-gear-oil-1-us-quart-0946-litre-1.jpg I switched to redline MT90. That didn't cure the o/drive problem but the transmission seemed to shift a little smoother...

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 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.This 70W80 GL-4 Gear Oil (SAE 5W30/10W30 engine oil viscosity) is designed for use in manual transmissions and transaxles. Provides excellent protection of gears and synchronizers and its balanced slipperiness provides a perfect coefficient of friction, allowing easier shifting. Popular in many BMW transmissions and in most Honda and Acura transaxle applications. Since most GL-5 gear oils for differentials are too slippery for manual transmissions, Red Line offers these products that ensure proper shifting while providing ultimate wear protection Popular amongst rotary owners Redlines MT-90 is a GL-4 Gear oil suited for your transmission/gearbox. Being GL-4 its ideal if your worried about the risk of some GL-5 oils prematurely/rapidly wearing or damaging your synchro’s (please note that Idemitsu Racing MTF 75w90 is a GL-5 that will not prematurely/rapidly damage wear your synchro’s)Something that cannot be said for all GL-5 Gear Oils when put in some transmissions like those found in the RX-8 Satisfies the gear oil viscosity requirements of 75W90, 80W90, 90 and motor oil viscosities of SAE 40, 10W40, and 15W40 Satisfies the gear oil viscosity requirements of 75W90, 80W90, 90 and engine oil viscosities of SAE 40, 10W40 and 15W40