Had a service last week, when I asked what oil was put in in case it needed top up I was told 5w30 semi synth.

I have got some 5w30 fully synth in the garage.

My question is will it be ok to top up with fully synth bearing in mind it will be a very small amount before the next service, at the most around a half litre?

Risk damage to a £20k car & too save £10  If I had all the money I spent on Beer!, I'd spend it on Beer!.

That wont be a problem snappa. semi synthetic is used as standard which is just a blend of mineral oil and synthetic oil which has never seen a hole in the ground. Fully synthetic is used a lot now in newer motors as the service periods can be extended as it does not loose its lubricating properties as fast as standard mineral oil does

As a chemical engineer, working for the lubricant sector for several years, I could tell you that generally is OK to mix semi synthetic with synthetic oil. Sometimes (rarely) the additives of the two different oils are incompatible leading in a faster degradation of the mixed oil. So is OK to mix oils only for a limited amount of time (1 month) e.g topping up until the next scheduled trip to the dealer.

Freelander 2 is using special A5/B5 fully synthetic oil, oriented for improved fuel economy. My advice is to keep a bottle of this specification for top ups. If necessary you can top up with semi or mineral oil only for limited amount of time. I say time because the degradation/aging of the oil, happens even in a car parked in the garage

A5/B5 for your I6, A1/B1 for his TD4

Synthetic only. Don't mix your oils. We are careful with the selection of different lubricants for different vehicles on the farm, and never mix anything. BUT, you do end up needing a storage shed as large as Halfords!

......my advice is to........ask yamaha-fan