Castrol GTX 20/50 availability

Are you just looking for a 20/50 mineral oil with a higher zinc content?

 Bicester Heritage is only about 10 miles away from me and driving in there to shop for oil is much more entertaining than a trip to Halfords or Euro Car Parts.

I have been using this in my 1970's Ford Essex engined car.

https://motorspirit.co.uk/product/heritage-20w-50

and for 1950s bikes I had some of this -

https://motorspirit.co.uk/product/penrite-enduro-25w-70

... Smith & Allen in Darlington. They do an excellent range including a high zinc content 20W/50. Best prices are through their ebay shop.

https://www.ebay.co.uk/itm/373537080268

Adam,hi. A word of warning, oils have changed as th common engine has been developed, some modern oils strip out yellow metals that our engines use to stay together. That is why th emphasis on "high zink" content in th above information for our engines. Also, some oils are containing "friction modifiers" which should be avoided in th chaincase/clutch area. This includes some ATF 10w. 

are completely absent in Tractor Universal oils, available as 10W-30 and 10W-40. Ideal for primary chaincases and forks.
These oils are for turbocharged diesel engines but also for hydraulics and oil immersed brakes (these do exist and not just unintentionally) and are cheap. All my agricultural machinery (does that include BSAs?) run on them.   

I use Valvoline VR1 obtainable from Opie Oils in Redruth, Cornwall. It is a very high zinc content mineral oil.

If you have any oil-related queries I have found Opie Oils to be very helpful and well-informed.

Some time ago I had a most enlightening discussion with them when trying to identify a fork oil that met the spec. given in the workshop manual for MZ's TS250: 45:1 mixture of "shock-absorber oil” and molybdenum disulphide; the oil specification is 8 to 11 cSt at 50º C".

Morris oils have a huge range including classic high zinc oils. Check morrislubricantsonline.co.uk ( the NOC used to have a discount arrangement with them. What happened to it?)

Oil blenders will openly tell you if the oil contains ZDDP as it can cause issues is modern engines. It is still widely used in many oils including semi and full synthetics. I tend to use the oils that are suitable for periods around 125C and above as they will still perform if stuck in traffic or at lights with long duration during busy periods. 

As I understand it the zinc content is to reduce wear on cams and followers. Modern synthetic oils that are 'low saps' may not have enough zinc content.
 I use fully synthetic oils to VW505 specification (VW507 doesn't have enough zinc) with an oil filter fitted. If the original oil specification for the bike was straight 40 I will use 5w-40 or 0w-40. I don't have any issues with cam and follower wear. Multigrades are much kinder to the engine for cold starts.
 The oils available now are far better than the oils available when our old Nortons were new, it's a shame the same can't be said for modern fuels!
 I used to use Castrol GTX but I would only use it now to run an engine in.