Understanding Gearbox Oil Ratings.

[TBCupraT]

Just had the gearbox in my LCR reconditioned by Bristol Transmissions.

The garage that re-installed the box put 2.5litres of 70w90 fully synthetic oil in the box.

I now have a problem crunching between 1st and 2nd when I first start the car from stone cold and goes away very quickly within 3-4 gear changes.

The garage contacted Seat Tech and had been advised that the correct fluid was in fact 70w80 fully synthetic and asked if I could get it changed.

My question is ( as I don't understand oil ratings) whats the difference between the two and does it sound plausible that the wrong viscosity fluid is causing my synco's to jam when cold.

 

[goble]

ones thicker one's thinner and yer wrong oil does cause it

 

[TBCupraT]

ones thicker one's thinner and yer wrong oil does cause it

Which is which?

Wrong? Is my assumption incorrect. Care to elaborate?

 

[goble]

The temperature range the oil is exposed to in most vehicles can be wide, ranging from cold ambient temperatures in the winter before the vehicle is started up to hot operating temperatures when the vehicle is fully warmed up in hot summer weather. A specific oil will have high viscosity when cold and a low viscosity at the engines operating temperature. The difference in viscosities for any single-grade oil is too large between the extremes of temperature. To bring the difference in viscosities closer together, special polymer additives called viscosity index improvers are added to the oil. These additives make the oil a multi-grade motor oil. The idea is to cause the multi-grade oil to have the viscosity of the base number when cold and the viscosity of second number when hot. The viscosity of a multi-grade oil still varies logarithmically with temperature, but the slope representing the change is lessened. This slope representing the change with temperature depends on the nature and amount of the additives to the base oil.

The API/SAE designation for multi-grade oils includes two grade numbers; for example, 10W-30 designates a common multi-grade oil. The first number associated with the W (again 'W' is for Winter, not Weight) is not rated at any single temperature. The "10W" means that this oil can be pumped by your engine as well as a single-grade SAE 10 oil can be pumped. "5W" can be pumped at a lower temperature than "10W". "0W" can be pumped at a lower temperature than "5W", and thins less at temperatures above 99°C (210°F). The second number, 30, means that the viscosity of this multi-grade oil at 100°C (212°F) operating temperature corresponds to the viscosity of a single-grade 30 oil at same temperature. The governing SAE standard is called SAE J300. The motor oil grade and viscosity to be used in a given vehicle is specified by the manufacturer of the vehicle.

Many new vehicles are marked to use 5W-20 oil (Honda, Ford and more recently Toyota). Some ultra fuel efficient and hybrid vehicles are marked to use 0W-20 oil.


[edit] Common multi-grade oils
Some of the common multi-grade oils are:

0W-20
5W-20
0W-30
5W-30
10W-30
0W-40
5W-40
10W-40
15W-40
20W-40
5W-50
10W-50
15W-50
20W-50


no wrong oil can cause poor gearchanges but will be o.k. when hot

 

[goble]

Gear oil is a motor oil made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It is of a higher viscosity to better protect the gears and usually is associated with a strong sulfur smell. The high viscosity ensures transfer of lubricant throughout the gear train. This is necessary since the devices needing this heavy oil do not have pumps for transferring the oil with only a portion of the lowermost gears bathed in an oil sump. This heavy oil can create viscous drag leading to inefficiencies in vehicle operation. Some modern automatic transaxles (integrated transmission and differential) do not use a heavy oil at all but lubricate with the lower viscosity hydraulic fluid, which is available at pressure within the automatic transmission.

Most lubricants for manual gearboxes and differentials are hypoid gear oils. These contain extreme pressure (EP) additives and antiwear additives to cope with the sliding action of hypoid bevel gears.

Gear oils are classified by the American Petroleum Institute using GL ratings. For example, most modern gearboxes require a GL4 oil, and separate differentials (where fitted) require a GL5 oil. It is important that purchasers check the oil against the vehicle manufacturer's specification to ensure it does not contain any aggressive chemicals that may attack gear components, such as the phosphor bronze used in many Japanese boxes.

API viscosity ratings for gear oils are not directly comparable with those for motor oil, and they are thinner than the figures suggest. For example, many modern gearboxes use a 75W90 gear oil, which is actually of equivalent viscosity to a 10W40 motor oil. Multigrade gear oils are becoming more common; while gear oil does not reach the temperatures of motor oil, it does warm up appreciably as the car is driven, due mostly to shear friction (with a small amount of heat conduction through the bellhousing from the engine block).

Fully synthetic gear oils are also used in many vehicles, and have a greater resistance to shear breakdown than mineral oils. They can improve the shifting performance of "difficult" gearboxes, where the excessive slipperiness of some mineral oils can impede synchromesh action.

 

[TBCupraT]

Thanks.

Well googled but still confused.

Is he right in saying that swapping to this grade will fix my crunch in second.


Sorry if I'm appearing blonde.

 

[goble]

sounds like a dodgy syncro but worth a go with the oil first

 

[Ricardo_Smooth]

changing the box oil wont affect it i'm afraid. 90 just means it wil stay thicker at the same temperature that 80 would (meaning 90 can run hotter) seems like the box re-furb place hasn't done a good job i'm afraid.