The wrong oil won't stop an engine from starting but the right oil will help it to last longer. Buy a respectable brand and there will be an application catalogue making recommendations for which of that brand's products are best in which engine.

It is little use changing parts and hoping. You need to be checking for a good quality spark and for the presence of fuel getting into the cylinder.
An engine needs compression and the right amount of fuel delivered into the cylinder and ignited at the right time by a spark of sufficient quality. An engine that doesn't start has at least one of those requirements missing. The challenge is to discover which...