When I bought my Vectra years ago the gears were real stiff in the mornings - 1st and reverse particularly difficult. Sometimes lights would change green and you'd still be sitting there like a twit fumbling with the gears as the folks behind sounded off their horns!!
I went to the local dealership and said as much and they just shrugged, laughing, and saying something to the effect of "what do you expect!!".
The box has no drain on these, so I sucked the oil out with a suction pump I bought for the purpose. It had the standard 75W.... whatever was in it.
I replaced it with Comma AQ3 Dexron 3. It has better corrosion inhibitors in it than regular 'box oil, is thinner in the cold, and has friction modifiers that improve the functioning of the sychros.
In fact, I replaced it twice, then again the next spring to make sure it was cleaned out as well as practical, as it was a 'suck' operation rather than 'drain' (the original oil I had taken out was disgusting! - Sealed-for-life 'boxes...hah! bloomin' madness...
).
Gear changes are now perfect, at any temp...
[...and before anyone suggests I shouldn't use Dexron 3 in a manual box, I say 'rubbish'. I've now run with that Dexron in it for the last 6 years and 80,000 miles. 'nuf said!...]
[And just to note further... I was discussing Vectra manual 'box oil with an indie garage a few years later and he said that Vauxhall/Opel had, subsequently, changed the box oil to another type that was, essentially, an ATF type.]
