The 308’s gearbox sits under the block and shares its lubrication with a set of transfer gears and the limited slip differential. It’s a complicated arrangement due to the transverse mounting of the V8 behind the seats, the gearbox being under the block and the differential being behind the block… compact, yes, but requiring a convoluted drive train.
Drive comes from the crankshaft to the clutch bellhousing (red). After the clutch, drive is transfered below the crankcase by a set of three transfer gears (green) and then back through the clutch bellhousing (red) via a lay shaft into the gearbox (blue) below the engine. From the gearbox, drive goes to the differential on the side of the crankcase (hidden above).
The fact that the same lubricant needs to lubricate both the gearbox and the differential means its a compromise for both roles. The synchros in the gearbox need a lubricant that does not slip too much since they need a certain amount of friction to slow or speed up one gear into the next during a shift. However, the differential needs a a lubricant that allows slip so that the rear wheels can roatate freely coming out of bends.
I use a Redline transmission oil that is recommended for both manual transmissions (and the synchros they contain) and differentials/transaxles, called 75W90NS. I have used this oil for 7 years now without issue. The usual second gear synchro problem improved when I switched to this oil but it’s still present when cold. There’s a school of thought that claims a combination transmission-diff oil like this causes premature wear of the synchros compared with a pure manual transmission oil. As with the engine oil debate, I don’t think I do enough miles for this to be an issue one way or another.
I found the above funnel at Walmart (sssshhhh its where I buy the exclusive, expensive oil for my Italian exotic!) and it is pefect for refilling the transmission oil. The transmission oil is drained from a plug (marked Cambio) in the sump next to the oil drain plug (marked Olio) – don’t get them mixed up! It is filled from the access port at the top of the transfer case (green) but to get the oil in there, it needs to be fed from the top, using a funnel lodged between the rear distributer and the coils.
The integrated, flexible extension tube on the Walmart funnel is idea for snaking down the side of the engine and into hole at the top of transfer gears. Pour in 5 quarts and that should be enough to run out the side gearbox plug when removed. Refit the side plug and pour in another half a quart for the transfer gears and you are done.
Disclaimer: 308restoration.com describes the restoration work I perform on my car and only my car. I am not a professional mechanic. The website content is presented for entertainment purposes only and should not by seen as any kind of advice, information, instruction or guidance for working on any other car. The opinions stated here are my own and no-one else’s.