Classic firewood hauler needs some help

[Easy Livin’ 3000]

My old Wheel Horse Raider 14 (early 1970's) badly needs a transmission fluid change. The manual calls for SAE 140 API Service GL-5. This must be an obsolete standard, as there is nothing with those specs available.

This is a manual transmission, and from what I can find online, gear oil is what I need. My question is, which gear oil would be a suitable substitute for SAE 140 API GL-5?

Also, when I have it in low gear range and am hauling a load of firewood down the slopes that make up my acreage, she slips out of gear into neutral, which adds some excitement, not in a good way. I am hopeful that flushing that old fluid out and replacing it will help with this issue. If anyone has any other thoughts about why this is happening, I am open to suggestions.

 

[greg13]

I doubt that an oil change will help jumping out of gear. Usually it is a wear issue.

 

[BrotherBart]

The tranny for the 14 should be in this manual. Maybe some help.

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[johneh]

Lucas Oil makes a 85-140 hd gear oil
(broken link removed)
Any Auto parts store should have

 

[Easy Livin’ 3000]

The tranny for the 14 should be in this manual. Maybe some help.

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Thank you, @BrotherBart ! I've received a lot of help here over the last year, but this takes the cake by a long shot. I have the owners manual and parts manual, and just yesterday afternoon was wondering if there was a service manual. Looking at the parts manual last night, I was certain that it would be a bad idea for me to try to take the transmission apart without a service manual. I'd get it apart just fine, but not back together. Problem solved, thanks to your kind help.