I'm out shopping with my wife and I'd like to pick up fluid for the transmission. What am I looking for ?

Regular Dino oil GL5 rated
Not synthetic

I used generic 80W/90W gear oil...

How much for an empty case?

Capacity? In total 2.5 quarts, but because you can't drain it dry while it's still in the car you can only really get about 2 quarts into it.

Regular GL5? Generic 80w/90w gear oil? Yes, but not any brand.

The consensus in the 911 world for many years (also with 901 gearboxes) is Swepco 201. Don't take my word for it. Take a look on an internet search. e.g.:
https://www.google.com/?gws_rd=ssl#q=recomm...ar+lube+for+901

I buy whatever generic 80/90W GL5 rated generic gear oil you can get at your local FLAPS.

I have only ever filled it after draining it dry.

So how does one go about checking the oil level?

Open up the top plug and stick a finger in it?

Oil level is checked on level ground ... pull top plug ... oil level should be even with the bottom of the hole. I fill till some starts running out and put the plug back in. You can get a gallon of gear oil with a pump on it from your local FLAPS (or used to be able to).

Is there an issue using synthetic?
I put in my trans about a year ago and it seems to work okay. Actually,It is a little hard to get it into 1st.

Do you guys think I should switch it out?

Swepco Is good but is it worth the cost...
Some here, including Dr Evil, believe the cheap stuff is just as good and the big thing is to change the oil more often.

I've used swepco and I still add about half a quart so it colours my oil blue, it makes it easy to tell the difference between a engine or tranny oil leak.

Swepco 201 grease is worth the extra cost IMO.

I ordered Swepco 201 because I couldn't find any good Dino oil locally. Every one I found had "limited slip" on the bottle, which I have read you should avoid.

Oh, and don't forget to remove the fill plug first, so you can be sure you can fill the transaxle.

What Non-Limited Slip dino oil are people using. Everything I come across is LS. I do see that Redline makes a Non-LS synthetic formulation.

Advance Auto (now CarQuest) carries Coastal 85W90. Just bought a gallon jug today. Note that it's labeled "Limited Slip Top Off Only" which means it has NOT got limited slip additives in it. Dr. E approves it also....

Kendall NS-MP

This is the only gear oil I run in 901's anymore. I can no longer find the cheap stuff locally, and this is specifically for hypoid gear, GL5, and non-limited slip diffs.

Do you want to explain your reasoning on that?

When I see post like these, all your going to get are all personal preference. It's like which beer do you like to down.
They all work, but if you work/ track your car most folks will use Swepco. Swepco is the gear oil that saved the 934 / 935 gear boxes back in the day.

they have trans fluid at the shoe store ...SOB who knew

Just pulling your chain Billy...you just got that car on the road not long back why you changing fluids already, what was in there before and didnt you put it in??

In my experience, Swepco is too slippery for the synchros to work properly. Not enough friction to slow down or speed up the next gear. Pretty much all of the big name brand gear oils you purchase at auto parts stores have friction modifiers in them that essentially do the same thing that Swepco does.

All of the auto parts stores in my area sell gear oil that meets 2 of the three requirements. It's either LG5, with hypoid gears, but meant for limited slip, or some other combination of that. These gearboxes specifically need gear oil that is designed for Hypoid gears, GL5, that has no friction modifiers for limited slip differentials. 901's are oddities in the automotive world, so most parts stores don't stock gear oil that meets these requirements. (In my area at least)

Will you notice a huge difference on a box with brand new synchros? Probably not. But when then box is a little older, and the synchros are worn, it makes a significant difference in the quality of the shift.

This is just my experience. I was changing gear oil at every oil change in an effort to figure out which one shifted the best. Between Coastal, Liquimoly Hypoid-Getriebol, Swepco 201, some ancient Castrol stuff I found in my dad's stash of race spares, and the Kendall, the Kendall has shifted the nicest.

I just replaced my Swepco two weeks ago.

I used Valvoline High Performance 80W-90 Gear Oil API GL4 & GL5 bought at Orielleys.

My transmission always shifted smoothly but ground a little going into reverse.
The transmission now shifts even better, (Toyota like ), with no grinding into reverse.

I was fine with the Swepco but couldn't find it locally.

I follow the good Dr. Evil's advice and go with GL5 rated plain old Dino. Sam's club has a case deal on Castrol Hypoy for 45 bucks.

With regard to quantity...the manual says 2.6 quarts. My transmission was just rebuilt and took nearly 3.

Guy at autozone wouldn't take no for an answer this morning when I said I wouldn't take syn as an acceptable substitute. I had to walk out mid sentence since he wouldn't stop reading off the computer screen and shut up

I've run Amzoil in mine since the 70's never had a problem.