MTFCA Forum

Went to the auto parts store to get fluid to replace on two Ruckstell’s. Found this in the correct weight but the label is confusing me. It says it’s rated GL5 for differentials but GL4 for non-synchromesh transmissions. So is it GL4 or GL5?

That's confusing to me. Many synchro transmissions have brass parts in them, and sulfated lubricants can cause corrosion, I'm told. I've never had any such problem. Non-synchro transmissions may or may not have brass or bronze parts, and may or may not be heavy duty applications. Put me down as totally confused.

It looks like the answer would be yes, I would try calling that 800 number and see there is an old guy there that knows what you are asking about. Most of the young ones that I talk to don't know and don't care. Good luck and let us know what you find out please. Dennis

I get Stalube GL-4 at NAPA.

When in doubt, order it from Lang's or another vendor who sells it. They wouldn't steer you wrong.

MichaelPawelek wrote: ↑

Sat Nov 06, 2021 1:02 pm

It says it’s rated GL5 for differentials but GL4 for non-synchromesh transmissions. So is it GL4 or GL5?

My interpretation is that it's both, depending on what application it's used for. Since you're wanting to use it in a Ruckstell, it's GL-5 for that application.

I've been wanting to agree along the lines of Jerry...it's dual rated by the maker depending on the application.

From my own experience, I don't think the old somewhat wives tale about yellow metal has anything to do with it.

When they redid the standards a while back, it was no longer a 'Mobil takes oil from deeper wells and refines it to a grade finer than Sunoco has the ability to process' sort of world.

Basic oil was now basic oil...a set formula for what it was and what it did by any producer...in theory interchangeable, but I'm too old school to allow that...became generic, and then things called 'performance enhancers' were added...and added...and added which resulted in the GL grades system where each number up is like a Starbucks flavor shot over the previous. There IS actually some internal rule of upwards and backwards compatibility associated with the GL system and its numbering system and thats what probably led to the dual rating on your jug. I just don't recall the rule off of the top of my head, but know one exists.

To the best of my knowledge, GL5 is actually GL4 with an additional dose of 'slippery' tossed in (not cling, slippery). How that relates to diffy's and non-synchro tranny's I haven't a clue...Guys who are more Motörhead than me can debate that one...lol. T's just wanted oil...and in the places it asked for special oils they seem to me to be looking for cling rather than slippery?

If you still feel concerned about having a dual rating oil...I think the solution is you just buy something single branded and have confidence that it is what it is?

Ford didn't seem to be too particular about what you put in the rear axle. Grease, oil, both, whatever. I'd think a fluid lubricant, like a 90W, would be best. I don't see how a # 2 or heavier grease could be expected to stay where needed in an axle, especially in cold weather. I'd also think that it would be difficult to tell how much you had in the axle, and that it would tend to pick up chips and debris that a fluid lubricant would allow to settle out. I believe a fluid lubricant would do a better job of dissipating heat from critical areas.