986 Forum - The Community for Porsche Boxster & Cayman Owners - View Single Post - Transmission Fluid

BrakeExpert · 12-18-2025, 06:10 PM

Bit of a supposition here but applicable.

As someone mentioned Honda, I've had a 97 Boxster and 97 Civic at the same time, had the same issue with transmission fluid/additives as I do with my boat. Not all 75W-90 oils are the same of course...many people on Hondaland with the 5-speeds heard that you can "cure grinds" with this miracle fluid called GM Synchromesh, which on the bottle, doesn't list its viscosity. These people read it online while nobody reported back after using it for a year, and here's why: Its for a Corvette transmission is suppose to have additives for an LSD. Thus people with Honda transaxles with an OEM style auto-torque biasing style (the ten gears) LSD will try it, and it will cure grinds for maybe 6 months. Then, you get a lovely noisy sound. This is because Honda's m/t fluids are a 10W-30 oil, considerably thinner than what the 986 or even corvette uses (or most transmissions actually), and there is an input shaft bearing that never gets any oil up there to its seal, same reason we have the IMS problem. Honda recommends their own fluid, but you CAN use 10W30 oil in its place for a short time (so says their service manual), and in my opinion, Honda makes among the most durable manual transmissions on any road car, hands down. But thus are the 'additives', as Honda uses its gear where the synchro touches as a deliberate wear point that is solid iron, its m/t boxes use a magnet as a filter (their automatics do this too, the drainbolt has a magnet you wipe clean).

Back to the 986 tranny. Same issue, as you mentioned, the chemicals blended into it are meant for the metal the bearings are made with, such as copper and Zinc.

Similar issue on my boat. I have a 1997 boat engine which is a chevy small block and the rockers are not rollers, meaning they rotate and was recommended to use oil heavy in zinc (25W-40). Because the year after this used roller rockers, you do not need to use this older oil and thus is recommended a GL-rated flud too. Due to new laws requiring boats to have cat converters, they would rather you not use so much zinc in this boat oil as the older oil is not catalyst friendly. I then must make a bit of a cocktail here as well, wanting the correct viscosity, but also wanting all that lovely zinc for my old-ass spinners. I therefore use Rotella T5 15w40, not the correct viscosity i wanted, but its said to have lots of zinc, and then ill add one quart of 25w40, because the zinc-filled oil is not around.

Hope this side-trip helped someone to understand and research the stuff they put into gear oils and that if you change a type of bearing or component to a different material, you may need to change oils to ones with other kinds of additives.

Edit: a lot of these viscosities changing or disappearing are usually due to more new laws against certain chemicals, so blame the lawmakers, not the dealership.

12-18-2025, 06:10 PM	#48
BrakeExpert Registered User Join Date: May 2014 Location: Atlanta, GA Posts: 100	Bit of a supposition here but applicable. As someone mentioned Honda, I've had a 97 Boxster and 97 Civic at the same time, had the same issue with transmission fluid/additives as I do with my boat. Not all 75W-90 oils are the same of course...many people on Hondaland with the 5-speeds heard that you can "cure grinds" with this miracle fluid called GM Synchromesh, which on the bottle, doesn't list its viscosity. These people read it online while nobody reported back after using it for a year, and here's why: Its for a Corvette transmission is suppose to have additives for an LSD. Thus people with Honda transaxles with an OEM style auto-torque biasing style (the ten gears) LSD will try it, and it will cure grinds for maybe 6 months. Then, you get a lovely noisy sound. This is because Honda's m/t fluids are a 10W-30 oil, considerably thinner than what the 986 or even corvette uses (or most transmissions actually), and there is an input shaft bearing that never gets any oil up there to its seal, same reason we have the IMS problem. Honda recommends their own fluid, but you CAN use 10W30 oil in its place for a short time (so says their service manual), and in my opinion, Honda makes among the most durable manual transmissions on any road car, hands down. But thus are the 'additives', as Honda uses its gear where the synchro touches as a deliberate wear point that is solid iron, its m/t boxes use a magnet as a filter (their automatics do this too, the drainbolt has a magnet you wipe clean). Back to the 986 tranny. Same issue, as you mentioned, the chemicals blended into it are meant for the metal the bearings are made with, such as copper and Zinc. Similar issue on my boat. I have a 1997 boat engine which is a chevy small block and the rockers are not rollers, meaning they rotate and was recommended to use oil heavy in zinc (25W-40). Because the year after this used roller rockers, you do not need to use this older oil and thus is recommended a GL-rated flud too. Due to new laws requiring boats to have cat converters, they would rather you not use so much zinc in this boat oil as the older oil is not catalyst friendly. I then must make a bit of a cocktail here as well, wanting the correct viscosity, but also wanting all that lovely zinc for my old-ass spinners. I therefore use Rotella T5 15w40, not the correct viscosity i wanted, but its said to have lots of zinc, and then ill add one quart of 25w40, because the zinc-filled oil is not around. Hope this side-trip helped someone to understand and research the stuff they put into gear oils and that if you change a type of bearing or component to a different material, you may need to change oils to ones with other kinds of additives. Edit: a lot of these viscosities changing or disappearing are usually due to more new laws against certain chemicals, so blame the lawmakers, not the dealership.