Transmission oil for MF-65

I have read the earlier post on mineral oil for the transmission/hydraulics/final drive f the MF-35 tractor and am curious as to what should be used in the system.

I recently replaced PTO output shaft bearing,seal and oil on my MF-65. Thanks to those that answered my queeries on that - the job went well. I used diesel fuel to flush out the sludge etc. that had built up in the system. The IT service manual that I have been using clearly says that a good quality 90wt gear oil is used in this system. Well, I used a 80W90 GL-5 oil when I refilled. Everything works fine, but my questions are:
1. Will this oil be harmful in any way, in the short term (say four months). I was thinking that if I have used the wrong oil I could easily change it out, however I would like to get some use out of it. It will also help to wash the system through even more.
2. Which is the best oil to use and why? I have seen comments on universal tractor fluid, hydraulic oil, Permatran oil etc. Could someone please elaborate a little more - I am a little confused on all the technicalities regarding the oil requirements.

Thanks for any and all comments.

JOhn