Sorry guys, another obsessive oil question.
What is the recommended viscosities in the UK?
I know that the US recommendation of 5W-30 is influenced by the need to wring out as much gas mileage as possible.
Currently, I am running synthetic 10w-40. Lower viscosities resulted in increased oil consumption, mostly during highway driving.
I am thinking of going to 5w-40 (Rotella?), less friction, better protection at start up and maintenance of low oil consumption.

 

How much oil are you using? Any leaks?

How many miles? Rather than change the viscosity why not try a fill with one of the higher milage oils -- we have had good results in the few we tried it on.


My XJR uses maybe 1/4 quart in 10k at the recommended weight

 

You are listed as being in CA.

In Mexico, the specific warranty requirement is 20W50 full synthetic.

That data point might have some effect on your choice.

 

5w40 Rotella is for diesel engines. The additive package, especially zinc, can prove fatal for catylitic convertor equipped engines. Bad idea.

If you're noticing a difference in oil consumption between a 5W- and a 10W- oil, there is something wrong with your engine.

 

Shell Rotella 15W40 is quite popular with the Porsche/BMW/Land Rover/musclecar crowd. That's a pretty big audience. The 15 base weight may be especially attractive to you. Shell claims the new "Triple Protection" formulation is xx percent better than the older "T" formulation for engine wear despite the lowering of the ZDDP additive percentage. It is certified as API CJ-4 and API SM. Anyone who does not believe that can look at the data sheet for themselves on the Shell site.

Starting at API SJ or so, and CI-4+, the allowable amount of ZDDP was progressively lowered. This was noted by builders of flat tappet cammed motors as the cause of cam lobe failures in newly built motors. Most enthusiasts would prefer the older formulation, but it is no longer obtainable. The current formulation is still an improvement over most passenger car oils.

The impetus behind the lower limits on ZDDP is the EPA 100,000 mile warranty requirement on catalytic converters.

It would seem to be a reasonable conclusion that the manufacturers chose a route that would ensure meeting the warranty requirement at the cost of shortened engine longevity. As long as the engine survives the power train warranty limits, there is not much of a downside for them, or at least, the lesser of two evils.

A user that opts for a high ZDDP content oil has to be made aware of the zinc/catalytic converter matter. However, as one writer put it: "I would rather replace a cat than rebuild my motor." Seems like a good trade. In any case, catalytic converters have lived for 20 years without failure on concentrations of ZDDP that are the same as found in the Rotella formulation.

The reason that there is a difference in oil consumption between the 5W and 10W is that the base weight is where the oil starts. Then, viscosity index improvers are added to achieve less thinning of the oil to meet the second set of numbers, eg. xxW30. The larger the spread between the numbers, the higher the required amount of VII content in the oil. The VII content is long strand polymers. When sheared, they become ... sludge.

In addition to flat tappet cams and lifters, ZDDP benefits other areas in the engine, such as cylinder bores. Any place where there is direct sliding contact at high pressure.

Finally, the diminishment of ZDDP may have contributed to the primary tensioner problem if the pictures of plastic timing guide parts on other makes are to be believed. Very similar patterns.

Unfortunately for those who have known and loved Castrol for years, it is no longer the same animal it once was.

 

But then, who is...