What oil should I use?

Hi everyone, my A1 has a newly assembled engine.
All seals and clutch discs are modern.
For the first use, I used mineral oil, which I plan to change after a few kilometers. My question is, should I stick with mineral oil or switch to synthetic oil?
Thank you.

We use 15w/40 diesel truck oil, it has the correct additives for our old school motors. Rotella T4 is a great choice.

 I also use diesel oil.  Some people will be along shortly saying that you MUST use MA certified oil, because otherwise your wet clutch will be damaged, but I don't believe that.  The low viscosity oils, 30 weight and below, often times have friction modifiers, and those can get to your clutch, but typically, 10W-40 or thicker oils won't, so the big worry about clutch damage is overblown.  

What does the manual or cap say?

 

The manual and cap were published before synthetic oils were widely available. The manual also says nothing about modern tires, blade fuses, et cetera. Many things have improved since the 1970s.

Any modern oil, without the extra friction modifiers, is in every way better than the best oils available when KZs were new. Synthetic oils are better at resisting shear and heat degradation than non-synthetics. This seems to me to be of special importance in air cooled engines that have the transmission in the sump.
Another consideration is the reduced levels of zinc and sulfur and the effect that has on flat tappet wear. 

I prefer a motorcycle or diesel specific full synthetic. 

I use Rotella T4 15W40.  I believe it's Jaso certified.

Rotella T4

I use Kawasaki KPO conventional 10w40 in my 74 Z1A.  I think it's made by Motul.  I've also used Rotella T4 15w40.

Scotty
 

The easiest way to choose a motorcycle oil, no matter where its used, is to buy it from a dealer in a container saying its for motorcycles with the weight thats noted in the manual or on the cap. Guys want to save a buck here and buy Walmart oil, diesel oil, etc its not necessary.  Personally I would never use non moto oil in my bikes or recommend it to others.

Based on the comments, I conclude that mineral, semi-synthetic or synthetic oil can be used as long as the viscosity of the 10W-40 oil is respected.

MLMG wrote:

Based on the comments, I conclude that mineral, semi-synthetic or synthetic oil can be used as long as the viscosity of the 10W-40 oil is respected.

The manual doesn't mandate 10W-40.  The chart below is from the 750 manual, but other manuals are similar.  I'd stay away from the thicker stuff if you are riding in cold weather, though.

 

I've had my A1 since 1979. I've always used Castrol 10W40 motorcycle oil. It's readily available to me and reasonably priced. I change it every 5,000 kms. The bike is at 88,000 kms now. My service manual states SAE 10W40.