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[DUFFEL]

Hi Leon
regarding using Pentite 25/75w oil on start up when cold the oil is only thin as in 25w but is equivalent to 75w when very hot ,they do all sorts of grade such as 20/60w or even 10 /70w .I don't think the Twins suffer with stripped oil pump drives owing to a relieve valve .I may be wrong there ??

[Leon Novello]

Hi Duffel: Yes you are correct, but, I would still be nervous about the strain on the pump when the oil thickens. From what I have read, there is damage done to the teeth from high-capacity oil pumps, but I don`t know anything about that.

[Bullet Whisperer]

For what it's worth, this thread is about a 250 unit motor and the oil pump drive is totally different and 20/50 will do the lot, no problem.

[Leon Novello]

Bullet Whisperer; you are quiet right, we do tend to `go off on a tangent` at times, but we are learning something new sometimes, I think?

[jefrs]

Duffel, you are correct that a 25W-75 behaves like a 25 when cold and a 75 when hot. The 75 is a viscosity value rated at 100°C because that is the temperature motors are assumed to run at. But the W for Winter number is not a true viscosity, it means the oil behaves 'like' a 25 when it is cold, oils having a number of necessary properties beside viscosity, and the 'winter' temperature covers a broad range from cold-start to well below freezing. Motors do not stay cold very long, the 'winter' number is the least important, the second larger number is the one it is running on once it has warmed up.

There are look-up tables for oil viscosity but 75 is a lot thicker than the motor should have even if it runs hot. Using a much thicker oil was an old dodge if the rings were worn and the engine was smoking.



When the 250 was born we had a choice of, Duckhams or Castrol really, and there weren't many grades, so 20W-50 was what you got.

Obviously an air cooled motor needs an oil that remains fairly well at its designated warm value over a wider range of temperatures than does a water-cooled one. Which is why I'm offering the opinion that a full synth will do the job better, because that is what they do, and are available in many grades (incl 20W-50). Whatever you chose, it is important to use the correct grade of oil.