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'74 350 Mark 3. I need to do an oil change. Since a full rebuild, I've being running Silkolene Comp 4 20w/50 and still have 1.5 litres left in the box but my local supplier only has 15w/50 available. What people think about mixing the two, then running just the 15w/50 after that?

Don't mention oil! Probably OK ( in my opinion).

Same opinion as Colin,

As long as they are both mineral based, I wouldn't have thought it would cause a problem for road use. Just don't mix mineral oils with castor (vegatable) oil based lubricants, that WILL end in tears!

Good health, BillR

I would'nd be too worried about your proposed different oil grade after your oil change....however.....I personally would not have run a freshly rebuilt engine on comp 4, if in the rebuild you have had a rebore or even new rings fitted they will take forever to run in on modern good quality (super slippery) oil, you really need running in oil for that job. If your rebuild was only bearings etc. you should be fine. My caution is from direct experience with my own 250 Desmo which had been run for nearly 2000 miles on a new borgo piston and a rebore......it was slightly burning oil and on a strip revealed piston rings still not bedded in, It had been run on Silkoline 'superma' 15/40. I changed it to Millers 'running in' oil and after a couple of hundred miles changed it to Silkolene comp 20/50 problem gone.
Cheers George

Hi George,

Good advice there! When I bought new pistons for my 750 Sport years ago from Vee Two Australia, the fitting instructions stated in capital letters ASSEMBLE WITH WD 40 ONLY. DO NOT USE ENGINE OIL ON PISTONS AND RINGS. I followed this advice and the rings bedded in perfectly. Apparently, this is what the Ducati factory did to bed-in new rings as well.

Cheers,

Colin

Hard to believe but some engine builders swear by leaving the bore and rings dry for a first start up (on the bench of course).
Cheers, George

blethermaskite wrote:I would'nd be too worried about your proposed different oil grade after your oil change....however.....I personally would not have run a freshly rebuilt engine on comp 4, if in the rebuild you have had a rebore or even new rings fitted they will take forever to run in on modern good quality (super slippery) oil, you really need running in oil for that job.

I'm glad i asked the question now, I wasn't too sure given the number of existing "oil" threads..... that's interesting about the running in oil and I've just ordered some. I'll run that before going back to the Silkolene.

Thanks
Phil

Another anecdote to back-up what George said: I remember reading an interview with Doug Hele, the wizard of Triumph tuning, way back when I was young, where he talked about assembling the Triple racing engines, and he said that the barrels were left outside overnight so that they gained a light rust coating on the bores, and then were fitted to the engines with one drop of oil on each side of the piston skirts. It was Doug Hele that came up with the idea of taking an epoxy casting of the combustion chamber and then having pistons made in that shape.


Cheers,

Colin

There were stories about speedway bikes with total loss oil systems, that an abrasive cleaning product called Ajax was added to oil to help hone the cylinder after a rebore. Of course, what might have been done in the competition world is not necessarily advisable for normal motorcycling.

an abrasive cleaning product called Ajax was added to oil to help hone the cylinder after a rebore.

Eek! I remember Ajax as a child. If I remember right, it was the colour of woodash, the texture of gritty sand and stunk of ammonia (maybe?). An interesting additive....