Usually the oil´s viscosity increases by the effect of water or glicol and decreases because of fuel dilution. Does anyone know about the possibility to happen the opposite, by the single effect of one of these contaminants? Do these contaminants promote ferrous particles?

I am not sure what you are looking for in the first part of your question but as for either fuel dilution or water/glycol contributing to ferrous particles the answer is yes they both can. Fuel dilution reduces film strength and in heavy load zones will increase metal to metal contact as surface asperities collide in the thinner oil film. Water or glycol can strip away the film entirely by interfering with the coating action of the oil. Water globules in the oil also can flash to steam in high load zones which blows away the oil film causing metal to metal contact.

As the first part of the question I was wondering if glicol and water can cause oil viscosity decrease and if fuel dilution can cause oil´s viscosity increase.

I think that the main reason for the viscosity decrease is not the effect of fuel. I checked viscosity as well as fuel content by gas chromatography in engine oil of my car, and 3-4% of fuel has negligible effect on viscosity of the SAE 5W-30 oil. Viscosity was nearly constant at 65 cSt/40°C. However, viscosity of operating SAE 10W-40 and 5W-40 oils was highly decreased at similar fuel content - viscosity of both oils goes from about 96 cSt or from 84 cSt to about 71 cSt/40°C, resp. The main decrease of viscosity was observed after the first 100 km from oil change. Main reason for that is the viscosity shear down and that is dependent on the type of viscosity modifiers.

Duarte,

I have not heard of viscosity increasing due to fuel dilution as this is inherently a lighter viscosity than the lubricant (there might be special cases).

One of the only times you might not see a drastical drop in the viscosity when experiencing fuel dilution will be if you have a severe restriction in the air induction system. This will result in fuel dilution and a significant increase in sooting.

Any contaminant that has an effect on the viscosity will inadvetently accelerate wear due to a breakdown in EHL and boundary lubrication regimes.

Have a great day!

Following my experience, the oil viscosity increase dramatically because of glicol penatretion (coolant) to the oil. But, you can detect it by checking the viscosity in 100 degree (Cel) and not 40 degree.