The old wive's tales are wrong on it not being good to change the transmission fluid if it has gone too long between changes. Even with an automatic. If a transmission fails after having its lubricant changed, it was about to fail anyway.
Your transmission is an AX-15 and Reverse gear will grind unless you do that momentary shift into a forward gear first before shifting into Reverse. Perhaps your technique isn't quite there yet, I'd keep at it... just the briefest of a momentary shift into any forward gear like 1st before shifting into Reverse (no need to lift up on the clutch) should do it.
Your AX-15 is a little fussy about its gear lube... it requires a GL-4 (not GL-5) 75W-90. A good GL-4 75W-90 is Redline MT-90 which you can get from www.summitracing.com
. You can read about MT-90 at http://www.redlineoil.com/product.aspx?pid=46&pcid=7
If you don't want to wait to order it, the manufacturer added synthetic 10W-30 engine oil to the AX-15's list of approved lubricants since GL-4 75W-90 can be hard to find on the local store shelves. No worries on the seemingly wide disparity between the synthetic 10W-30 and GL-4 75W-90 gear lube viscosities. They are actually very similar viscosities, engine oils and gear lubes are measured on two different viscosity scales.