Petrol vs diesel engine oils?

[SMOKEU]

I've read online that diesel oils tend to be thicker than petrol engine oils, but if you compare the same weight oils, are they the same thickness? For example, does a 20W40 petrol engine oil have the same viscosity at the same temperature as 20W40 diesel engine oil?

[Grumph]

Yep - but the diesel oil will have a different assortment of additives, mainly to add shear strength. Zinc content is often the major difference - modern car oils have had the zinc content reduced of recent years which is not good news for the load bearing ability of the oil.

[Flip]

Not quite right.

Most motor oils have dual petrol and diesel ratings. I am a great fan of mixed fleet lubes, have been for years. They usually come in bulk and do not have any fancy packaging and I would rather pay for good oil than some expensive bottle and some flash holographic label.

Oils in recent years (after SG) have been formulated with low metals so that they do not pollute the cats on modern European motors. The metals are part of the detergents, just like soap that is made of fat and potassium or a hydrocarbon and a metal, the detergents are made up of a metal like zinc and a oil-hydrocarbon.

Diesel motors need a good strong detergent package. Diesel motors pollute their oils, and after too longer service the oils can become acidic. Some of the heavy diesel oils are called high ash (strong detergent) and are used in the biggest diesel boat motors. These motors often run on MDO and Bunker oil which is a crappy smelly black diesel fuel. Medium ash oils are usually big truck oils. They are called ash because it is the metal oxides that are left after all the oil has been burnt from a sample of oil in a lab.

Bikes tend to kill their oils because the oils run in gear boxes and the gear boxes are hard or chop up the VI (Viscosity Index) additive so the oil thins with use. Thats why bikes usually change gear better with new oil. The VI additive is the gorilla snot that is used to make a straight 20W weight oil behave like a 50 weight oil at high temperature ie 20W50. The VI additives are big stringy molecules and are not very shear stable so break down under mechanical work.

[Paul in NZ]

Not quite right.

Most motor oils have dual petrol and diesel ratings. I am a great fan of mixed fleet lubes, have been for years. They usually come in bulk and do not have any fancy packaging and I would rather pay for good oil than some expensive bottle and some flash holographic label.

Oils in recent years (after SG) have been formulated with low metals so that they do not pollute the cats on modern European motors. The metals are part of the detergents, just like soap that is made of fat and potassium or a hydrocarbon and a metal, the detergents are made up of a metal like zinc and a oil-hydrocarbon.

Diesel motors need a good strong detergent package. Diesel motors pollute their oils, and after too longer service the oils can become acidic. Some of the heavy diesel oils are called high ash (strong detergent) and are used in the biggest diesel boat motors. These motors often run on MDO and Bunker oil which is a crappy smelly black diesel fuel. Medium ash oils are usually big truck oils. They are called ash because it is the metal oxides that are left after all the oil has been burnt from a sample of oil in a lab.

Bikes tend to kill their oils because the oils run in gear boxes and the gear boxes are hard or chop up the VI (Viscosity Index) additive so the oil thins with use. Thats why bikes usually change gear better with new oil. The VI additive is the gorilla snot that is used to make a straight 20W weight oil behave like a 50 weight oil at high temperature ie 20W50. The VI additives are big stringy molecules and are not very shear stable so break down under mechanical work.

Shit - thats a great explanation... thanks...

I run Penrite Diesel or Gas in the old triumph as it has a wet clutch and HPR30 now has a friction modifier which made it slip - seems to work great.

[avgas]

truth be told I am running a diesel oil flush through the bike right now. It seems a little thinner than most motorbike oils. Even though its a 10w50 feels more like 5w60.
Dunno - but the detergents are great. I have got rid of all my false neutrals, and now it engages the actual neutral like a dream while running.

[Flip]

What kind of oil are you flushing with? Its probably a great oil.

I don't yet trust the very modern oils after SG because well they have been formulated for new low emission motors and none of my vehicles are this type. The features that light diesel oils have are the features I believe I want in a motor oil. A good strong detergent package so that the soot, dirt and metal particles end up in the oil filter. Strong buffer chemicals so the oil stays neutral PH. Not too much VI chemicals so that the oil does not thin out too much with use.

The thing I really don't want is for the oil manufacturer to skimp on the oil so that they can sell the stuff in a fancy pants one liter bottle. I am not interested in the performance of the bottle.