I'd go belt-and-braces and be generous with the straight 40 oil on the big ends too, that should give plenty of protection at first start-up.

I'm pretty sure I've seen reference to people packing oil pumps with this so I guess it should be OK?

Not planning to do that, but I'm putting it on the conrod big end bearings.

I'll use whats left of my SAE40 motor oil on other surfaces,, and then if the engine actually runs, I have some diesel-only 10W40 that may or may not be OK in an SI engine long term (havn't found any info on that) but I'd guess should be OK as a short term "flush", so the vaseline (thermally stable?) residues wont be around for very long.

AFAIK diesel engine oils simply have more detergent additives than petrol oils, so might dislodge accumulated surface carbon/varnish.

I can't think of any reason not to put 'diesel' oil in a petrol engine. Or vice versa for that matter, tho oil for diesels presumably contains some kind of additives.

Back in the day I remember priming oil pumps with vaseline to help prime the oil pump but I can't remember using on engine bearings etc. I used to use a clean oil can to wet rocker/tappets(pre hydraulic!), bores, bearing surfaces &oilways especially in crank lube holes and con rods. This also kept things lubricated during assembly so I could check all was well at each stage of assembly. Where possible I used to fill filter with oil ready for action.

I'd be worried about using on some surfaces especially near bearing lube holes as it might cause a slight delay re oil feed to the bearings through tight lube holes. Probably not an issue though.

Edited by Big John on 31/05/2023 at 22:19