I ran a normal (mineral ) 80-90W gear oil in the Saginaw box in my Monaro.. no problems
think back then, it was Castrol brand

I just checked... Penrite do exactly that these days and is the recommended oil for the Saginaw
per their 'selector' drop down

https://penriteoil.com.a.../gear-oil-80w-90-mineral

well, try other brands 80W-90 mineral oils and try one without the additives

pps.. you DO NEED extreme pressure additives in gear box oil

Edited by user Saturday, 4 September 2021 7:49:03 AM(UTC)  | Reason: Not specified

In the '70s and '80s we used to run Castrol EPX 80W/90, which Castrol used to list as equivalent to GMH AL93 SAE90 weight, the recommended Saginaw oil.
Castrol now seem to shy away from EPX for that application and recommend Castrol Universal 80W/90. Perhaps EPX has grown some additives that are undesirable.

Bill Tuckey described the Saginaw gearshift as stirring a crowbar in a bucket of rapidly setting concrete, and thats not far wrong - they aren't renowned as the slickest shifting gearbox around. If you are expecting slick light bang gearshifts from a Saginaw you are doomed to disappointment.

If you are having synchromesh problems in all gears, and the box has been rebuilt recently, I would have a good look at the synchro keys, and the rings themselves. The accuracy and quality of these items in the recent lot of rebuild kits has been very poor.
Unless you've got sewing machine oil or treacle in the gearbox, that's a more likely explanation than a few points of oil viscosity.

Back in the day I had quite a nice collection of chipped and broken gears to prove it too.

I wouldn't be surprised if it was simply an adaptation off another North American vehicle. GMH had similar designs here using the same shifter that was normally mounted on the gearbox but mounted remotely. One was the early LH big block Torana where an Aussie 4spd was fitted behind the Opel 1.9 but it sat forward of where it did behind a 6cyl or V8. So GMH mounted the shifter to the tunnel and used longer shift rods. The other I can think of was CF Bedford with Aussie 3 or 4spd. The standard Aussie 4spd shifter was used remotely with long shift rods. I bet there were obscure Chevrolet, Pontiac, Buick, Olds etc vehicles that used exactly what we had in HK-HG.

Thanks guys for all the advice but I have owned and driven this gts 327 for the past 25 years and it was good prior to the refresh,the issue is either the oil or the blocker rings as these are two of the 4 things changed, the other two being the bearings and seals.
The remote shifter was a typical GM north America thing at the time, the 67 Camaro's had a similar setup but GM moved to the tailhousing mount in either 68 or 69 as did Corvette. HQ muncie went to the tailhousing deal as a result.
I'm going to change the oil as a first move, I too liked the old Castrol EPX but the current stuff reads poorly, formulated to cover a few deleted oils including lim slip oils.