Can't get reverse gear.

Have a 2000 Rodeo Manual V6. Have started to notice that some mornings I can't get into reverse. Even lifting the clutch doesn't always help and I grind it sometimes to get in. Once warm it's stiff but works. About 5k miles ago I changed the transfer and gearbox oils to 5W synthetic motor oil. The manual even suggests this. But does anyone think that the thinner oil is the problem? Not helping the sync? Forward gears appear OK at the moment.

Maybe try a conventional 5w or 10w-30 & see if that helps? I dunno how the syncro's come into play in reverse, but some can be sensative to the type of oil.
G/luck
Joel