Picking an appropriate gear oil viscosity for my light diesel truck

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May 23, 2018
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Per my last thread, I got some good answers but I would like to ask in a different direction: this time with picking appropriate gear oil viscosity. My vehicle is an Isuzu dmax (diesel powered with 130kw and 380nm of torque) with a 5-speed manual synchromesh transmission.

The book specifies a 5w30 besco transaxle fluid (no available data sheet, but being a 30 weight i presume its KV100 10~12Cst) or 15w40 (14-15Cst) engine oil. Currently I am running Amsoil Synchromesh fluid 5w30 with a 10.1 Cst. Ran like a dream with Amsoil MTL for 4 years / 63k kilometers until recently when it started shifting notchy both cold and when warmed up.

Would a 75w80 gl-4 with 7-8 Cst be enough to protect my manual transmission considering its a light diesel truck that's occasionally used for hauling boxes? Or should i stick to a 75w85 since it falls between a 30 and 40 weight engine oil (that's specified by the book).

If a 80 weight won't cut it I would probably use the same engine oil I use in my engine--Zic 10w30 CI-4 which is a stout full saps diesel oil with 10.9 Cst.

I am in a tropical region if it matters, no freezing temperatures. warm 25C-37C temperatures all year round.
 
How much are you saving 30$?
Half actually, I need 3 liters for my trans. Comparable gear oil such as the besco transaxle 5w30, Mitsubishi mtf gl3 75w85 costs half of what AMSOIL charges nowadays. Being an MLM with annual price increases isn't just sustainable for me in the long run.
 
Half actually, I need 3 liters for my trans. Comparable gear oil such as the besco transaxle 5w30, Mitsubishi mtf gl3 75w85 costs half of what AMSOIL charges nowadays.
I gave a suggestion in a PM and I have no idea what comparable fluids are available in the 10-12 cSt range in the Asian market. If cost is a factor then stick with the Misubishi GL3 or the besco.

Commodities everywhere have gone up in price and Amsoil isn't the only one so let's not single out Amsoil. Amsoil's MLM's don't set the prices.
 
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Half actually, I need 3 liters for my trans. Comparable gear oil such as the besco transaxle 5w30, Mitsubishi mtf gl3 75w85 costs half of what AMSOIL charges nowadays. Being an MLM with annual price increases isn't just sustainable for me in the long run.
Surprised there is MLM in Philippines.

4 years, hot climate, warm engine, lots of shifts - not that bad! It probably eventually was under 10cSt and got notchy.....my guestimate

Anyway I would have recommended MTG 75W-90 GL4, so look for something in that range. Around or under 15cSt @ 100°C, I bet works best.
 
The book specifies a 5w30 besco transaxle fluid (no available data sheet, but being a 30 weight i presume its KV100 10~12Cst) or 15w40 (14-15Cst) engine oil. Currently I am running Amsoil Synchromesh fluid 5w30 with a 10.1 Cst. Ran like a dream with Amsoil MTL for 4 years / 63k kilometers until recently when it started shifting notchy both cold and when warmed up.

...I am in a tropical region if it matters, no freezing temperatures. warm 25C-37C temperatures all year round.
MTF's can shear down in viscosity over 4 years, so it doesn't sound as if you need a lower viscosity fluid but a higher viscosity fluid such as Amsoil MTG


or HPL's Manual Life 75W90.
 
I gave a suggestion in a PM and I have no idea what comparable fluids are available in the 10-12 cSt range in the Asian market. If cost is a factor then stick with the Misubishi GL3 or the besco.

Commodities everywhere have gone up in price and Amsoil isn't the only one so let's not single out Amsoil. Amsoil's MLM's don't set the prices.
Thanks for replying Molakule, if cost wasn't an obstacle I would've bought another batch of Amsoil's MTL, but budget dictates I explore other options at the moment.

Surprised there is MLM in Philippines.
Yes Pablo there are a lot of MLMs here, Unilever, Avon to name a few;Amsoil's the one relevant to me though. Years ago their prices were highly competitive and I always bought their oil--5w30 diesel ss, 10w30 diesel turbo truck and 0w20/5w20 ss for my gas cars.

Years ago a quart of 0w20 SS was 580 Philippine pesos and now its 880!! Synchromesh was Php900 four years ago. now its Php1200. The economy is also in a slump due to global economy and bad decision making by local leaders. Its just life, but I had to adjust accordingly.

4 years, hot climate, warm engine, lots of shifts - not that bad! It probably eventually was under 10cSt and got notchy.....my guestimate

Anyway I would have recommended MTG 75W-90 GL4, so look for something in that range. Around or under 15cSt @ 100°C, I bet works best.
Wouldn't a thicker oil make it harder for the synchronizers? I do like me some more protection but I am treading lightly if I would sacrifice comfort.

Other users who used 15w40 in their dmaxes from other forums reported harder shifts

MTF's can shear down in viscosity over 4 years, so it doesn't sound as if you need a lower viscosity fluid but a higher viscosity fluid such as Amsoil MTG


or HPL's Manual Life 75W90.
Do all shear stable gear oil shear under in 4-5 years? So by going a bit thicker I'm giving myself a safety margin, is that correct?
 
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Wouldn't a thicker oil make it harder for the synchronizers? I do like me some more protection but I am treading lightly if I would sacrifice comfort.
Not in your meteorological conditions. I don't know what you mean by "comfort."

Other users who used 15w40 in their dmaxes from other forums reported harder shifts
That's because engine oils do not have the proper type of friction modifiers that is needed for an MTF.

Do all shear stable gear oil shear under in 4-5 years? So by going a bit thicker I'm giving myself a safety margin, is that correct?
I have found shearing in MT's can be indicated in less than two years.

Again, if cost is an obstacle then comparable gear oils such as the besco transaxle 5w30 or Mitsubishi mtf 75w85 are your choices.
 
Not in your meteorological conditions. I don't know what you mean by "comfort."
My apologies, shift feel and less vibration felt in the cabin. I've noticed over the years that as the gear oil gets used up I feel more vibration from the accelerator pedal under highway speeds at 5th gear (80~100km/h). Replenishing with fresh gear oil immediately mitigates said vibration.

The dealer used to fill my dmax with Shell Spirax 75w85 gl-4, and in 20,000km the shifts felt loose (no resistance going in gear, like shift knob is jello) and I felt vibration from the pedal, so I immediately replaced the gear oil. 20k gear oil looks pristine though.

That's because engine oils do not have the proper type of friction modifiers that is needed for an MTF.

I have found shearing in MT's can be indicated in less than two years.

Again, if cost is an obstacle then comparable gear oils such as the besco transaxle 5w30 or Mitsubishi mtf 75w85 are your choices.
Thank you, I have inquired with different dealerships. Besco (Php 4,150 for a 4L can) is more expensive than Redline (1qt for Php980, or 925 with the upcoming xmas discount). I bought the mitsu 75w85 gl3 (stocked but sealed) from a guy who bought the wrong gear oil for his Mitsu. Got it for half the price bnew from the dealer :D and a third of what besco and redline costs. Will post how it fares in my isuzu box.

Researching more I've found that in South Africa, Isuzu users there tout Castrol BOT402 as the best mtf transmission there is for these boxes. Specifically designed for GM/Opel transmissions--which isuzu share a lot of technology with. BOT402 is 75w85 gl4 with a KV100 13 cSt and dyed red. Sadly I can't source it in this region.
 
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