Viscosity drop at Cat C engines

Dear Rene
Yes, we checked the fuel existance, flash point etc.
It is very interesting, that the same type of oil on other Cat engines family as 3508 etc, provide very good viscosity after 330 HR (only 5% drop) .
On C type Cat engines, the viscosity drop 15% at 330 hr,and after 500 hr (Cat recomendation), the viscosity drpo 25% compare to a fresh oil.
We are running 10 units of C engines, and all of them indicate the same problem. It does not make sense that all of them are suffering from the same problem.
Dan

I am curious as to which C type engines - C15, C16 or C18??? Has the sump been increased in size to achieve 500hr oil drain intervals?

C-32 and C-15
The sumps capacity is the original . I do not see any reason to increase the capacity. I am following Cat instruction.

I would be curious to see how another engine oil would perform. Have you recently changed brands of oil? It might be worth trying a different brand in one of the trouble engines and see if the UOA results show that the problem has gone away or if your viscosity drop problem continues.

Dear Michael
The same oil and the same brand. latetly, I sampled the C engines and the other on 250
Hr. The Viscosity on the C engines is 10% less.
The data is very clear.

Have you looked at antioxidants - ZDDP, phenol , amines?
Although they would not be responsible for change in viscosity - unlike shearing of VI improver - they could tell you severity of environment oils experiencing in thinning/nonthinning engines. Also what are Cu numbers. Any deposits showing up?

Poosible reasons for viscosity loss are fuel dilution, polymer shear(VI improver shear)and top ups.
Is there a possibility that top up oil is not the same viscosity as oil change oil?
Would like to see vk results at 100C and VI and additive levels (for potential wrong oil additions TO4 etc)
VI will drop with polymer shear.
Suprised your lab has not suggested Vk100 and VI