Thanks everyone for the advice. It's been useful.

Sorry for the delay in returning to reply.

It's 88. When you say you've been running on unleaded, what octane rating?

I drive a Volvo 340 (petrol engine). When four star disappeared I moved over to Lead Replacement Petrol with no problems. But now that seems to be being phased out.

Does anyone have any experience with additives, or any other suggestions for what to use in the 340?

I'm wary of using Redex after ...