Engine Oil

Castrol XL 20/50  is probably what Norton would be reccomending  if they were  around. If you have a proper filter I would use a fully synthetic oil in the engine  which would last  for a longer service interval  say 5000 miles.

I use Castrol 20/50 in my Mk3 Commando, which of course has a filter and works well.

But my 650ss, 99 and Dommie Racer, all with no filter, run on Castrol straight 40.

This works well for me, so I don't need to fix it.

PS: If you were to fit a filter to your 650ss then 20/50 the way to go, in my opinion of course. And don't forget the Esso Ethanol free petrol too, while you can still get it. (Supreme+ with red label)

by the way, there is NO definitive answer..

 (deduced from reading many threads.)

but there are good guiding principles, some of which have already been pointed out.

Agreed there is no definitive answer but there is an independent test here https://www.accessnorton.com/Oil-Tests/NortonOil.php

Castrol XL 20/50 performed well in this test.

Andy

I think it would be difficult to go very wrong with any  of  the oils availiable  as the  specs are  much improved on what was  around  back in the old days.  I mention  fully synthetic after  having the experience of a very overheated engine ( wrong carb setting)  while running on 10/40 fully synth oil.,which would generally be thought to be a bit too thin.  After it cooled down everything went back to normal ,no apparent  damage. Most impressed.  All I need now is  a fully synth 20/50 for about £30 for 4 litres  .  Some hope.!!  

I hate to pour cold water on anothers efforts, but I have grave reservations about the value of this work. It perports to relate to valve train conditions. to do this you need to reproduce the load conditions in the engine, (fluctatiog with valve lift), the entrainment speed of the cam/ follower contact, the flash temperature increasee due to the friction, the metallurgy of the contact. You then need to demonstrate repeatability at different performance levels using industry reference oils of known field performance. Even then you have the problem of reproducing the chemical environment in the engine,blow-by gases, fuel dilution and  build up of acidic products. This study appears to satisfy none of these requirements. I  recommend that you go with any high quality 20W50.

Lex (How are you mate?) was absolutely correct in stating that there is NO definitive answer and I was clear about what works for me, and has worked for a very long time.

Charles, I would agree about using high quality 20/50 BUT as this grade appears to always  include detergents, then without the fitting of an external filter, oil changes would need to be so frequent  (Depending on the condition of the engine) as to be not cost effective, never mind the down time.

Cooler running E0 petrol must help with keeping oil clean?

Don't be afraid of oils which contain detergents. they arre useful in keeping your engine clean. The concern seems to that the accumalated sludge may block up your oil ways so try a good flushing oil first. Classic Castrol 40 does not come cheap compared with a good 20W50 oil of superior performance specification. The enemies of your oil are, soot from incomplete combustion, deposits on hot surfaces due to oxidation,acids due to oil oxidation and sludge due to water+incomplete fuel combustion . Your newer type oil will offer better protection against all these.

When they became available, multi-grade oils were recommended in some of the Norton manuals and contemporary handbooks.  For temperatures above 32F it was 20/50 and 10/30 for below 32F.  It's listed on Roy Bacon's "Norton Twins" for example. Back in the late 1960s Duckhams Q 20/50 was the most universally available so we all used it!  I took my 99 around the clock so that the speedo was banging against the stop (About 30 mph then! LOL!) and never had any trouble.

Yes, there is a definitive answer. The oil you use must contain ZDDP additive, it can be found in straight oil, multigrade, even full synthetic oil.

The oil manufacturer will let you know what products contain it as it can severely damage some exhaust systems. 

A lot of sense in this discussion and we are all being very nice about it, a good thing.

Just had my Poor Man's Manx out today after a long lay up, or should I say 'lockdown?'

I run my singles on GP 50 (No Issue) but I noticed on the tin, not applicable to 30 or 40 that this is a light detergent oil. No worries but worth knowing. Only my slimline model 50 runs up any decent sort of mileage among  the singles, running it well into autumn but topping up with SAE 30 from late Summer.

But if they all had an external filter then I would run 20/50 in all of them. except for my 16H. 

Back in the 70's I I'd stick Q in everything, nice to have a choice now, at least with oil...    

Hi Roger,

You could mix semi synthetic with mineral oil but not mineral oil with full synthetic. Not sure about semi synthetic with full synthetic?  Charles, what do you say?

I don't think it is the same as changing from Castor to mineral or synthetic where an engine strip down is suggested necessary after Castor.

Good luck with whatever you choose and let us know if you have issues. I hope not...