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Help! Rover P3500S gearbox lubricant????


Jedi Warrior
Working on a Rover 4-speed manual gearbox from a P3500S. (NOT the later LT77)

manual is calling for 20W50, but I've read that the spec has been changed to 75W90..... and /or ATF..... I've seen recommendations for Castrol SMX-S, but that isn't available in the US.

I have no hair left to pull.

Has anyone here had any experience with this? I really would prefer to run Redline Gear lube in this thing, but I don't want to risk screwing up an already dicey gearbox.....


Jedi Hopeful
AT fluid is generally not used as a direct replacement for manual gearbox lube in any application. The lubricity and other specs are different between transmission types. AT fluid is essentially hydraulic fluid and in some applications is actually spec'd for power steering and other hydraulic uses. A manual transmission is a gearcase more in line with a differential, although with extra moving (sliding shift mechanisms and synchronizers)within. 20w-50 is a motor oil by design and as such does give up certain properties better handles by labeled heavier gear lubes. This does not mean they actually are thicker, just how they perform.

Keep in mind that UK oil specs and even Japanese oil specs are sometimes based on oils that are not available here - case in point from experience - Kubota, a well-known manufacturer, has consistently called for a certain weight gear lube in transmissions and front axles which, if subbed by a heavy gearbox oil is too heavy, instead their US spec is a 10w hydraulic oil. It took them years to rewrite teh manuals to even just add the proper oil as an alternate spec, even though that's what they leave the factory with. I think the original specs were written for whale oil.

Back to your application. You could use the 20w-50 motor oil as listed but since some evidence suggests alternatives then a proper transmission lube is in order. Do not use Redline regular gear lube, but instead their MTL/MT-90 transmission gear lube. There are differences, as I've said, between duty requirements of an oil involving helical cut gears, synchronizers, and shifting rails as opposed to more simple spiral cut hypoid diff with or without limited slip.


Jedi Trainee
<span style="font-size: 20pt"> </span> <span style="font-weight: bold"> </span> <span style="color: #3333FF"> </span> Sorry for not getting on here sooner.........Yes use Redline MTL.....MT-90 is for consistanly higher temps.........20W50 is the lowest you should use but could be enhanced with MolySlip Gearbox treatment.....I've used this before with good results but now with the REdline....this is the way to go...........Drain your old fluid first of course
Good Luck


Darth Vader
You can also go to www.castrolusa.com and enter year and model of your vehicle and Castrol will answer you on what lubrication is needed in any part of your vehicle. And it will be the latest fluids they have.


Freshman Member
The 4 speed P6 box definitely used engine oil, like 20w50 without issues. I must have done several 100k miles in my 3 cars, always with a very light oil. I doubt they would last on 75w90 heavy type oils. The B version was an uprated internally over the 4 cyl version, with an oil pump. Or get a R380 5 speed box, a prop shaft out of an auto and go for it. A little work needed for the rear box mount, but not hard.