Changing Transmission Oil

[blinkwatt]

How much has anyone paid for a transmission oil change on a 2.5L(manual of course)? Is this a DIY for a novice?

[MNBoxster]

Hi,

Easy DIY project. You'll need:

Parts:

• 17mm Allen Drive (for Fill Plug)
  
  16mm Triple Square, anti-tamer socket (or Hazlet 2567-16 12 Pt. Bit - .5" Drive - avail. on eBay or http://www.zdmak.com/wbstore/main.asp?action=PROD&PROD=HAZET256716&CTMP=1 ) (for Drain Plug).
  
  2.25 Ltrs. (2.4 US Quarts) SAE 75W90 Hypoid Gear Oil
  
  1 Stiff Piece of Mechanics Wire, or Coat Hanger w/ 1" bent to a 90° Angle

Procedure:

• Raise Car securely on proper Jack Stands
  
  Remove Plastic Splash Shield and Alloy Diagonal Braces
  
  Remove Fill Plug (17mm Allen Drive)
  
  Place Catch Container under Drain Plug and remove Drain Plug
  
  Clean both Fill and Drain Plugs
  
  Once Drained, replace Drain Plug - Torque to 25Nm (18 Ft./Lbs.)
  
  Add new Gear Oil through Fill Plug (Handpump and Tubing Helpful), about 2 Quarts.
  
  Then, insert Bent Wire, bend in downward orientation and pull back out, look for Oil level to be 11mm (7/16") below Fill Opening, keep adding Oil until this level is reached.
  
  Replace Fill Plug and Torque to 25Nm (18 Ft./Lbs.)

Differential shares the manual Transmission Oil supply, so no need to change or check level there.

Hope this helps...

Happy Motoring!... Jim'99

[Brucelee]

I am thinking Mobil 1 full syn gear oil 75W-90.

Jim?

[MNBoxster]

Originally Posted by Brucelee

I am thinking Mobil 1 full syn gear oil 75W-90.

Jim?

Hi,

You certainly could use Mobil 1, but the benefits probably don't outweigh the increased cost, and since it's rated GL-5, it may not be the best thing to use.

This is because Hypoid Gear Oil is used mainly for the Differential which uses Hypoid Cut Gears. Because of the sliding contact that Hypoid gears make, (Hypoid gear teeth appear twisted) , their Hydrodynamic Contact pressure is higher. To be suitable for use with Hypoid gears, a lubricant must be capable of resisting these higher pressures and is rated as EP.

A Manual Transmission won't usually contain Hypoid gears, so it
doesn't need an EP oil. But, in Transaxles (as in the Boxster) where the
Crownwheel and Gearbox share the same lubricant, Hypoid Gear Oil is needed.

Hypoid Gear Oils contain greater amounts of Sulphur or Sulphur/Phosphate compounds to withstand the higher Hydrodynamic Pressures and prevent Shearing. They have a typical Rotten Egg smell to them, especially after some service time.

One problem with these compounds is that they can chemically react with the parts in the Transmission, especially Yellow Metal Parts such as some Synchros and Bearings. I cannot find any information on the composition of the Tranny's internals, but if they are using any Brass, GL-5 Lubes can cause premature wear. This is why I would not use the Mobil 1. Instead, I'd go with a GL-4 Rated Hypoid Gear Oil - same Lubrication and Compressability qualities, but does not attack Yellow Metal Parts.

Mobil 1 touts their benefits as withstanding Higher Temps (not an issue here), Longevity (again, the Tranny is a sealed system and not subject to Dirt, Moisture which shorten the Lubricants Lifespan) and lower Viscosity (again, not an issue unless you're in Alaska, especially if you drive passively until the Gearbox has warmed up - which you should).

Personally, I just don't see the need and the fact that there could be some Brass in the Tranny causes some concern. It seems to me that possible accelerated wear to the Tranny and higher cost are real negatives. There are lots of GL-4 rated Lubes out there which will work just fine - I'm not sure there is a better in this case.

But, since the Tranny is sealed and unfiltered, it is prudent to change the Gear Oil every 3rd year to remove any of the metal shavings which will accumulate. Not more often because Gear Oil is actually better if a little aged - the Fresh Additives will not be especially good for it. Hope this helps...

Happy Motoring!... Jim'99

[JackG]

Redline MT-90 is a GL-4 rated 75W90 synthetic gear lube.

Royal Purple also has a Max-Gear synthetic that is GL-4/5 rated.

Both are claimed to not be corrosive to soft metals. Maybe these are possibilities?

[MNBoxster]

Originally Posted by JackG

Redline MT-90 is a GL-4 rated 75W90 synthetic gear lube.

Royal Purple also has a Max-Gear synthetic that is GL-4/5 rated.

Both are claimed to not be corrosive to soft metals. Maybe these are possibilities?

Hi,

They certainly could be. I have heard nothing but good things about the Royal Purple Line of products, and I use RedLine MT-90 in my Lotus Esprit.

The Lotus uses a Citroen SM Gearbox (modified), and it's really at the extreme of it's HP Rating in the Esprit. It uses Brass Synchros and so a GL-4 is called for. The Tranny shifts smoother and it is a must for any power-modded Esprit. In fact, Lotus introduced the Esprit SE because the Citroen Gearbox was no longer available and so they switched to a Renault Box which could take more HP, so Lotus redesigned the Head and aded a Chargecooler to up the HP to match the new Box.

If the Boxster uses hard metal synchros, there's no problem at all, but I've never been inside one and so cannot say for sure. But, GL-5 has been reported repeatedly to have attacked soft metal parts. GL-4 does not.

One other thing about the Synthetics is that they invariably make the Tranny noisier. This is because of their lower viscosity, and for some people, this may be a consideration.

I love synthetics, but not necessarily in every application. If the benefits cannot be utilized, it seems silly (to me at least) to pay a premium for the stuff. So long as the Oil level is kept at spec, a gearbox rarely fails because of the Oil it's using, except when Hypoid Lubes are used. Otherwise, most Lubes out there are really adequate. Gearboxes fail mostly because of consistent Operator Error...

Happy Motoring!... Jim'99
Happy Motoring!... Jim'99

[insite]

Originally Posted by MNBoxster

One problem with these compounds is that they can chemically react with the parts in the Transmission, especially Yellow Metal Parts such as some Synchros and Bearings. I cannot find any information on the composition of the Tranny's internals, but if they are using any Brass, GL-5 Lubes can cause premature wear. This is why I would not use the Mobil 1. Instead, I'd go with a GL-4 Rated Hypoid Gear Oil - same Lubrication and Compressability qualities, but does not attack Yellow Metal Parts.

helpful post. FYI, i had the opportunity to look inside a 986 5-speed this week. the synchros are indeed brass.

[blinkwatt]

Anyone here used Redline MT-90 with a 6-spd gearbox? If so how was it?

[MSull788]

Red Line MT-90 is good, GL4 90 weight hypoid.

[Adam]

Changing the gear oil certainly couldn't hurt especially if it's never been changed. That would be the first thing I would do. Just use a quality synthetic 75w90 gear lube. Mobil 1, Amsoil, Redline, Royal Purple, Swepco...take your pick.