Engine oil

[viva boy]

What engine oil should we be using ?
5/40
10/40
15/50 etc ??

[1972nail]

20W50 is the simple answer. Don't go for cheap supermarket ones as they don't have enough zinc to protect our older engines.

I've recently been using Miller's Semi Synthetic Classic Sport 20W/50 in my OHC engine and it's behaved impeccably during the recent hot spell when mineral oils would have given problems. Mineral oils have a lower 'shear' temperature than semi or fully synthetics. If the shear temp has been exceeded once the oil looses high temp viscosity for ever. OHC engines have a problem that oil temp is normally comparitively high and in warm weather after a fast run the shear temp of mineral oils can be exceeded. This causes lowered high temp oil pressures until the oil is changed. I haven't experienced that problem with the Millers Semi Synthetic oil. It's worth noting that OHV engines are not nearly as prone to that problem unless highly tuned or flogged nearly to death!

The Millers Classic oils also have enough zinc for the much needed cold start up protection. Older engine designs had lots of steel to steel rubbing contact, such as camshaft to follower and this depended on zinc to provide the lubrication when cold, modern oils don't have enough to give protection. Modern engines are designed to have tolerance of the lower zinc levels in modern oils.

So, in short a good specially designed 'Classic' 20W/50 mineral will be OK but for added high temp oil longevity go for a Semi or Fully Synthetic Classic 20W/50.

[viva boy]

Many thanks for the info maybe a member of admin can make this an information post on the OHV engine section

[Colin]

Many thanks for the info maybe a member of admin can make this an information post on the OHV engine section

That's a good idea. David certainly knows what he's talking about.

[Bob perry]

I found the 'Classic Oil' 20/50 supplied by Halfords extremely good.
When poured it was a lovely golden/green colour similar to the old Duckhams oil.
Might be the same supplier for all I know, and was in a Green and silver metal can under a Halfords label.
Going back for more.

[1972nail]

Comma make a good mineral 20W/50 and that's who probably make Halfords. I've used it for years in our Minor without problems and the Halfords one before that. I used the Comma oil in the Firenza OHC engine but found it began to break down when it got really hot. When restarting the hot engine immediately just after a long run it would take quite a long time to build pressure. Sitting on a forecourt ticking over with no oil pressure for a couple of minutes can seem like an age....

A change over to Comma 5w/50 Fully Synthetic Motorsport oil solved the problem, instant pressure (as instant as on OHC can be!) whatever the the temperature, but it doesn't have enough zinc for older engines, hence the change to Millers 20w/50 Classic Sport Semi Synthetic. There is also a fully synthetic in the Millers Classic range but in my opinion it's not really needed in a mildly tuned road car.

As I said in my post further up, a good 'Classic' formulation mineral 20w/50 will suit an OHV engine perfectly, however, in my experience, a mineral oil isn't the best available option nowadays for the OHC engine.