I think I need to top up the oil in my MY03 WRX. On the invoice for my 1000 mile service it says that the garage used Shell Helix Plus. I didn't check this before I bought some Motul 15/50. Will it matter if I put the Motul oil in my car with the existing Shell oil?

Just in case it matters I've done 3750 miles & its nearly 3 mth old.

 

Bttt

 

It won't matter too much if you put a small amount of the Motul in, but to be honest it's not really the best option at the moment.

If I were in your shoes, I'd buy a litre bottle of the Shell to keep it topped up for the time being.

Then, wait another 750 miles or so, and change the oil and filter. Put some semi-synthetic in of around 5w/40 to 10w/50 (Mobil Super S, Motul 200V etc.).

Change again for more of the semi-synth at around 7,000 miles, then, finally, once you're over 10K, change again and put the Motul 300V in.