This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
One of the easiest tasks to perform on your 911 is to change the transmission oil. The 911 transmission is what is known as a transaxle. It includes all the standard components of a normal transmission, plus an integrated differential. This design is possible because of the rear engine design of the 911. The transaxle design is more compact, and theoretically lighter in weight since you don't need a dedicated differential.
The differential and the transmission both share the same lubricating fluid. It's very important to make sure that the fluid in your transmission is at the proper level, otherwise your transmission will experience significant wear. The synchro rings and sliders (see Pelican Technical Article: Rebuilding First Gear on the 901 Transmission) all depend on a slick surface in order to match speeds when shifting. If your transmission is low on oil, the wear on these components will accelerate significantly. In addition, shifting the car will be more difficult. One of the first things that you should check on a 911 that is having problems shifting is the level of the transmission oil. Keeping the differential and all the associated gears well lubricated should also help increase your fuel mileage.
The transmission oil also helps to keep temperatures down inside your transmission. The engine is one of the primary sources of heat for the transmission as it conducts and radiates through and around the points where the engine and transmission are mounted. The transmission also creates heat itself as the gears and synchros turn within its case. Keeping the transmission fluid at its proper level helps to mitigate heat problems. Having a large reservoir of oil to spread the heat throughout the transmission helps to keep temperatures down. On some of the higher performance Porsche transmissions, there is even an external transmission cooler that operates similar to the engine cooler.
I've always been a believer in sometimes over maintaining, especially if you can afford it. I check and change my fluids pretty frequently. The manual recommends every 30,000 miles, but doesn't explicitly state 30,000 miles or X years. It just states that it depends on the amount of driving and on the driving habits. Oil and wear and tear items should be checked frequently and depending on the driving habits and the conditions of where you're driving the car, the fluid should be changed more frequently. Check your owner's manual for more details on the scheduled requirement for your year 911. Again, this number is a rough estimate, and may vary depending upon your use of your 911 (track vs. street). There are many moving parts in the transmission, and they have a tendency to drop small microscopic metal particles into the oil. Specifically, the synchro rings wear down slowly over time, each time you shift. While the transmission bearings are not as sensitive as the engine bearings, they can still exhibit wear from these particles in the oil.
The 901 transmission has two plugs for filling and emptying the transmission oil, located on the side of the case. The 901 transmission was used on 911s from 1965 through 1971. The 901 transmission requires a 17mm hex key to remove the M25x1.5 plug. On the 915 transmission, used on 911s from 1972 through 1986, the plug is the same, but located on the bottom of the transmission instead of the side. With the G50 transmission, used on 911s from 1987 on, the locations of the plugs were similar to those on the 915, but the plug type changed from requiring a large hex key to requiring only a 19mm socket.
If you are simply checking the level of oil in your transmission, start by removing the top filler plug on the side of the transmission. This is the plug that you add fluid to. When you have the plug removed, take your finger, and stick it inside the hole, point it towards the ground, and see if you can feel any fluid in there. Make sure you do this when the car is cold, and parked on level ground. If you can feel the fluid level with your finger, then your fluid level is about right, or perhaps will need only a little topping off.
If you cannot feel the fluid level, then you will need to add transmission oil to the case. If you are planning on changing the oil, then remove the small plug on the bottom of the transmission case (on the side of for the 901 transmissions). It's a wise idea to try to empty the transmission oil when the car is warm, as this will make the oil more viscous and it will flow out easier. Make sure that you have a drain pan capable of handling at least 5 quarts of transmission oil. Check the fluid in the pan to see if you see any unusual metal pieces, or grit in the oil.
While the fluid is emptying out, you can use this time to clean out the drain plugs. The bottom drain plug should have an integrated magnet in it that traps metal debris. Using a cotton swab or a paper towel, carefully clean out all of the black debris and particles that may have found their way in there.
While the plug on the bottom is magnetic, the top plug that shipped originally from the factory wasn't. A simple upgrade is to install one of the bottom magnetic plugs into the top filler plug location. The added magnet can't hurt the transmission, and may help to remove some additional particles.
Replace the bottom plug on the transmission, but don't tighten it too tightly (24 Nm or 17.6 fl-lbs maximum). These plugs do not have a tendency to leak (transmission oil is thicker than engine oil). If it does leak later on, you can always tighten it a little more. Now, add transmission oil to the case. The best method of doing this is with an hand operated oil pump. These are available from most auto parts stores and attach to the top of the plastic transmission oil bottle. They work very similar to the liquid soap dispensers you find in most bathrooms. Pump the transmission case full of fluid until it just starts to run out the filler hole. Replace the filler plug and clean up the few drips that might have run out of the hole. Tighten down the filler plug in a similar manner to the drain plug.
In many cases, generic transmission gear oil will suffice perfectly fine. However, for those 911 owners wishing to have the best of everything for their car, there is Swepco 201 Multi-Purpose Gear Lube. This gear oil is excellent for transmissions, and many Porsche owners swear by it. The current rumor is that adding Swepco 201 will prolong the life of many transmissions and also help to postpone a costly rebuild. While this can hardly be proven, a lot of owners will agree that using Swepco 201 creates a difference that you can feel while shifting. For the G50 transmission (1987+), I recommend using a synthetic blend that is more suited to its Borg-Warner synchro design.
Shown here are the filler and drain plug for the 915 transmission used on the 911 from 1972 through 1986. The drain plugs on the other transmission models are similar as well. You need a 17mm hex key to remove the plug from the transmission. The bottom plug is magnetic, whereas the top plug was originally not. A worthy upgrade is to replace your top plug with one of the lower magnetic ones.
The gold standard for transmission oil is Swepco 201. Used by racers and Porsche owners worldwide, many would swear that it gives new life to an old worn out transmission. Customer's stories from the parts supply world seem to indicate that using Swepco on a transmission with worn synchro rings can prolong its life and fend off that costly transmission rebuild.