Hi Jeff,  

I use Rotella 5w40 T6 full synthetic.  Yes, it is a diesel oil, and as such it includes a heavier load of zinc (ZDDP) than an oil formulated to modern gas engines. The zinc is, as I understand it, beneficial to our flat tappet motors.  You might spend a bit of time researching.  This is a good starting point: https://www.bobistheoilguy.com/ Then search for articles on ZDDP and classic cars. The T6 Rotella is about 1200 PPM. Mobil 1 auto oil is about 750-900 PPM.

Have fun,

Jim

> On Nov 9, 2018, at 11:52 AM, Jeff Palmer <w.jeff.palmer@ICLOUD.COM> wrote:
> 
> Hi - trying to get one last oil change in before parking the van for the winter. 2.1 litre engine rebuilt about 30k km ago. Normally I use 20W/50 in the summer and I “believe” my mechanic has been using 15W/40 synthetic. I can only find diesel 15W/40 synthetic at my FLAPS. This is not the same, is it? Is this the right weight of oil for me?
> 
> Thanks 
> Jeff in Winnipeg
> 
> Sent from my iPhone