The FSM is wrong. It originally said GL-3. GL-5 has more extreme pressure (EP) additives which are usually sulfur based that erode the brass synchros over time. You actually need some friction for the synchros to work properly and the EP additives interfere with that. 75W-90 gear oil is pretty close to 10W-30 motor oil viscosity-wise, and actually thins out less with temperature. When I had an AX-15, I ran MTL90 which does not use sulfurs and is consider "yellow-metal" safe. I used NAPA GL4 right after I rebuilt it, planning to change it after break-in. It shifted so bad I thought I buggered the rebuild, but decided to put the MTL90 in anyway. It was like night and day, and the MTL shifted fantastic. I've never run 10W-30, but there are a lot of guys on here who swear by Mobil1 10W-30 and supposedly some of the Toyota trucks that used the AX-15 spec 10w-30.

 

this is what redline’s website recommends for 1990 XJ AX15: https://www.redlineoil.com/75w90-ns-gl-5-gear-oil

would you stay away from that?

 

I think that would be a good choice. It's a non-sulfur GL-5 without friction modifiers.

 

I’ve sprayed several vehicles with Fluid Film and Woolwax and I personally like Woolwax Black the best. Seems to hold up better in my opinion as it is thicker than Fluid Film. Use a heat gun to warm up the can contents before pouring it into the sprayer they sell with the kits.

IIRC, FF and WW are owned by the same company…

 

I went with the Redline MT-90. Seems to be one of the most popular and no one seems to have any issues. The pump I got says it is NOT for use with the Redline bottle so I’ll have to transfer it to an empty quart of oil bottle. I assume I don’t need to rinse out the oil bottle first? It would just be whatever oil (clean) is left in the bottle after I empty it out.

 

I usually go thru thru the brakes...check lines,pads,shoes and usually go ahead and flush it out with new fluid unless I know the previous owner did it.

 

I saw a bunch of videos on Surface Shield being the newer and better Fluid Film. Made by the PB Blaster company. I sprayed it on a couple of weeks ago, very sticky so little dripping and virtually no odor but use a face shield and a mask anyway. Took about 2 hours to do a pretty thorough job. Figure 4 to 5 of the aerosol cans. Available at your local Home Depot for ~$11 a can.