It has to be coincidence because there is no relationship between changing the engine oil and the transmission operation.
Now the logical side of me knows that the most common cause of a manual transmission that won't stay in gear is internal. But the "I don't want to tear it apart" side says, "hey maybe it's something simple." Sure thing Bruce. So anyway I decide to change the gear oil, check the clutch hydraulics and adjustment. (Just as an aside when I put the 5-speed in about three years ago I don't think I put pure gear oil in.)
I decided that while I was working on the truck I would do some long needed minor repairs. (You all know where this is going.) I needed to replace one wheel stud on the right front, the ugly, rusty windshield wiper arms should be painted, and I thought I'd check the brakes. Well the wheel stud didn't get replaced; who knew that Toyota made the front hub and rotor a two piece, bolted and pressed together? Not me. And I do not have a press so that job will have to wait - forever. I did paint the wiper arms.
With the truck up on four jack-stands I noticed some fluid on the inside of the left rear wheel. A quick sniff said it's gear oil. Great an axle seal is leaking. Not really a problem; I've replaced hundreds of axle seals.
So I pulled the drum and of course the brake shoes were soaked. I stripped off the shoes and hardware and realized the backing plate is the bearing retainer. So I had to remove the wheel cylinder, which I did. As I'm holding the wheel cylinder this brownish fluid starts leaking out of the dust caps. Sniff, sniff. It's not brake fluid it's water! Somehow water got in behind the dust caps. I pulled the dust caps back and there was so much sludge in there ... Now I need wheel cylinders (I never replace just one).
Off I go to the parts store - brake shoes, wheel cylinders, axle seal, and brake cleaner. Got everything back together - the seal was probably the easiest part of the job - and decided to reuse the brake shoes. I know what you're thinking - he's an idiot. Way back when I worked on big trucks. We routinely used to clean oil soaked brake shoes because of cost and availability. What did you think all that brake cleaner was for?
Everything is back together and the brakes work fine. The transmission? It's the same of course. I really don't want to take it apart, it's going to tie up the truck for about a week (I am slow sometimes).
There's still so much that I could fix on the truck. I suppose I could buy a new(er) one but there's a part of me that enjoys getting my hands dirty.