What oil to use in a flathead?

An old topic back to the top/////////////
A couple of comments.
90 weight is usually a gear oil. A similar viscosity to SAE50 engine oil.
Oil companies measure engine and gear oils on a different scale. (Measurement in centistokes)
Gear oils do not have additives for combustion residues.
Cheaper oils usually have a lower additive mix. You get what you pay for.
The choice of what is the 'right' oil for your motor is complicated. If you have a lot of $$ in your engine it is wise to look after it.
Re additives, I was told a long time ago that adding additives was a bit like adding a spoonful of baking powder to Grandma's cake recipe. If the original recipe was lacking it will be improved. If correct, it will be made worse.
A better idea is to combine the cost of the oil and additive and buy a better quality oil
I feed my flathead a diet of 20/60 made specifically for classic engines, with zinc etc. Oil is cheaper than parts.

 

Yep... it looks like oil technology has moved on since this thread started.